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Using Wikibooks/Deleting, Undeleting, and Importing

Deleting Pages
Deleting pages is a task that administrators do not engage in lightly. There are two ways for a page to be deleted: Through the Requests for Deletion (RfD) and through speedy deletions.

Requests for Deletion
At Requests for Deletion, Wikibookians can nominate books and pages that don't meet the project's inclusion criteria. The Wikibooks community will discuss the request and, if they agree, the page(s) can be deleted. Over time, a number of fancy templates have been created to help manage these discussions. While it isn't required to use these templates when discussing, or when closing the discussion, it certainly helps. Plus, if you don't use the templates, somebody else may come along and correct you.

A request for deletion is not a majority voting process: nothing on Wikibooks truly is. There is only one real rule that needs to be followed: When in doubt, do not delete. However, this rule does not require 100% agreement in order to delete a page. An admin must not only tally the comments in favor and opposed, but must also read their rationales. Using all this information together, the admin must come to a firm conclusion. In other words, the community may express doubts, but the deleting admin must have none.

Given the freedom of admins, there is a slim possibility that an admin may choose to delete a nominated page, despite the community being in favor of keeping that page. This is permissible, but the admin should be prepared to defend the action with solid reasoning. While possible, it is highly recommended that admins avoid this kind of vigilantism, except in the most exceptional of circumstances.

Speedy Deletions
Some pages can be deleted quickly, without having to be nominated on Requests for Deletion. These are known as speedy deletions or simply "speedys". A page only qualifies as a speedy deletion if:

 * 1) It is vandalism or spam, or other useless information that is not part of a book
 * 2) If it is a user page, and that user has asked the page to be deleted
 * 3) If it is the user page of a vandal who has been blocked forever
 * 4) If the page is just a repost of a page that has already been deleted
 * 5) If the page is part of a book, and the authors of that book have agreed to delete the page
 * 6) If the page has been nominated for RFD, and the community has decided to delete.

Use Template:delete for speedy deletions.

If the book does not fall under one of these categories, it should be nominated at Requests for Deletion instead of being speedy deleted. Notice that rule 1 above implies that the page history contains only vandalism or spam as well. If a page was useful, and was replaced with vandalism, it should be reverted instead of deleted. Before deleting, always check the page history.

The fifth rule isn't technically a "speedy", but it highlights an important point: The authors of a book, if there are several of them, form a mini-community within Wikibooks. These mini-communities are empowered to make decisions about their books. In fact, we like to think that the authors of a book will understand that book well enough to make deletion decisions about pages in that book. Nobody knows a book better than its author. If an individual page from a book is nominated for deletion, it is common practice to move that discussion to the book's talk page. When the authors there reach a decision, the page can be deleted as a speedy. In short, it's not really a speedy because there is a discussion about the page, and a discussion, etc. Once the decision is made, however, an admin can treat the page as a speedy, block the deletion if the content is found significant (especially if no consensus process occurred) or anyone can change the speedy into a general community discussion by re-tasking the "speedy" for a request for deletion process.

The sixth rule also doesn't sound like a speedy at all. Once a book has been slated for deletion, sometimes the Impending Doom template is placed onto the page, to give the authors time to save a copy of the book. This is done as a courtesy. The Impending Doom template puts the book into the speedy deletion category, and technically any admin can delete that book as a speedy upon seeing the template.

Books that don't meet the inclusion criteria, or that don't appear to be appropriate textbooks for whatever reason can be nominated for deletion at WB:RFD. The community can discuss the issue, and once a general agreement is reached, the page will be either kept or deleted (or sometimes merged with other work, or transwikied to another project).

Nominating for Deletion
When a page is nominated for deletion, mark it with rfd, and create an entry at WB:RFD with the reason for nomination. Once the community has discussed the matter, and consensus has been reached (or not), the nomination can be closed. This makes it clear that discussion is over, and clarifies the outcome. If the community decides to delete, replace rfd with Impending Doom. If the community decides to keep, remove rfd and place rfd-survived on the talk page.

Listing Nominations on the RfD Page
On the RfD page, list the name of the book or page using a link (i.e. Pagename ) in the header of your rationale. Then in the message box, type your rationale for why you believe the item in question does not satisfy the inclusion criteria. Make sure to sign your post with four tildes ( ~ ).

Purpose the RfD Page
A discussion at "Requests for Deletion" is not a "vote" in the sense of a democracy, majority-rules decision. Rather, debates are made until the community arrives at a consensus about what should be done about the nomination. If no consensus can be made, a user will generally attempt to synthesize a compromise solution.

Several templates are in use for different types of comments. The keep template is used when you are in favor of keeping content, and del is used if you are in favor of deletion. It is highly recommended that you leave comments near your opinion because the decision is not based on majority vote but quality of argument and community consensus.

For general comments that are meant to be neutral, use the comment template.

Viewing Deleted Pages
Admins can view deleted pages, and they can also view a list of a users' deleted contributions. In general, information in these pages should not be made public, unless there has been a specific request for the information (such as on WB:RFU, described below) and the page in question does not contain any sensitive or inappropriate information. Admins should be careful and cautious when viewing deleted pages, and making those pages available to other users.

Undeleting Pages
It's less common than page deletions, but there is a mechanism for undeleting pages as well. If a page has been wrongfully deleted, or if a community member wants to reexamine the deletion decision, the page can be nominated at Requests for Undeletion. If the community agrees to undelete the page, an admin can undelete it.

Many times an admin will temporarily undelete a page, so that an author or reader can extract information from it. It is left to the discretion of admins whether and when to do this. A page that has been undeleted temporarily can be deleted as a speedy when it is no longer needed.

Importing
Often a book can take advantage of work done on Wikibooks' sister projects. In this case, the page should be imported and merged into the book. This is better than a simple cut and paste operation, as it brings not only the wiki markup, but also the page history. Preserving a page's history is required in order to respect the GFDL copyright terms.

Requesting an import
Non-administrator and non-importer can ask for an import on this page.

Using Imported Data as the Basis for a New Book or Chapter
If the page is to be used as the starting point, it can simply be imported into Wikibooks by using the Import tool. The page is imported into Transwiki name-space, and then moved to whatever the new book's name will be.

Incorporating Imported Data into an Existing Chapter
If the page is to be merged with an existing book chapter, things get a little more complicated. The first step is to import the page as outlined previously. The imported page can be moved from the transwiki space to the book chapter - using the same name as the existing book chapter. This will cause the existing chapter to be deleted with the imported material taking its place. The administrator can then undelete all the previous material. At this point, any editor can take over and edit the chapter to incorporate old and new material, cutting and pasting to the heart's content. If the content was moved before this process, you'll want to copy the wiki markup of the existing page before beginning, and use it to replace the new version after undeleting.

To summarize:
 * Import page
 * Move imported page on top of existing chapter — this will delete the existing chapter
 * Undelete chapter material
 * Edit (or restore to its original state)


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Using Wikibooks/Vandalism

Vandalism
Spam is when outside groups try to use Wikibooks as a vehicle for advertising. Spammers, often automated spam programs, insert links to external commercial websites on various pages.

Vandals are not financially motivated. They disrupt the project by adding inappropriate pages and content.

Types of Disruption
There are several common types of disruption that have occurred at Wikibooks in the past. Some of these types have been mitigated through technical measures. Some have simply become less popular for vandals over time. Others are still in occasional use. In general, Wikibooks has very low levels of spam and vandalism, especially compared to what it once had. However, it's a problem that never goes away completely and that our administrators and vandal fighters should be prepared for.

 * "Friends of gays" vandalism: This is the type of vandalism where one immature editor makes inappropriate remarks about other people they know from offline. Often, it's considered an easy way to put an insult online in a place that looks "official". This can be comments like "John is gay" or "Sarah is a whore", and are usually inserted without much fanfare on a single page. The goal is to insult the target not to disrupt the Wikibooks project, so this type of vandalism is usually found in unpopular and rarely-visited pages that only the attacker and victim are likely to be looking at. Since the goal is not to disrupt the project, these types of vandals are not serious and do not need to be dealt with harshly.
 * Image vandalism: This is where an inappropriate or pornographic image is uploaded to Wikibooks (or pre-existing images are used inappropriately), and it is added to several pages. Particularly difficult forms of this vandalism inserts the image into popular templates, which causes the inappropriate image to be displayed in many places. This type of vandalism used to be very common, however new technical measures make it less appealing for vandals. New accounts must wait 4 days before they can upload an image, precisely because of this type of vandalism.
 * Page move vandalism: Page move vandalism is when the vandal renames a page to something inappropriate. A tame example would be moving Main Page to Main Page on Wheels!!. This type of vandalism used to be very common, like image vandalism. However, new technical measures such as fast reverts of page moves and requiring new users to be autoconfirmed before moving pages have helped reduce this.
 * Personal Attack vandalism: This type of vandalism represents a personal attack on one or more editors here at Wikibooks, and can be comprised of image vandalism, page move vandalism, or other types. Common targets are typically admins and active vandal fighters who have piqued a personal interest among the vandals. This type of vandalism usually occurs in the User: and User talk: namespaces, especially on the user and user talk pages of the person being targeted.
 * "Is this for real?" 'vandalism': This is a very benign type of vandalism where new and incredulous users post test edits on pages to see if wiki is real. This type of vandalism, which isn't even called "vandalism" in most cases, is less common than it used to be just because society has become more familiar with wikis and people are less surprised by it. These types of edits should be redirected to the Sandbox, or to a user's page, instead of occurring in the content or discussion pages. These users are often just being curious, and shouldn't normally be blocked because of it.
 * Page blanking vandalism: This is vandalism where a user deletes all or substantial portions of the content from a page. Removing all content will be obvious in Special:RecentChanges where the system will automatically insert a message that the page has been blanked; removing lots of content creates the (-14,528) alerts. Page blanking is not always vandalism, and sometimes it's a perfectly normal part of the editing and authoring process. Keep an eye out for these kinds of edits, and don't hesitate to revert them and start a discussion if you are worried.
 * Edit summary vandalism: Using disruptive and inappropriate edit summaries, as part of scandalous edits or even as part of edits that otherwise look normal, is still considered vandalism. Those bad edit summary messages can be trapped in the edit history of the page forever, or until it is removed by an admin or a user with oversight access.

There are dozens of other types of vandalism, and many repeat vandals have particular signatures that can be used to identify their handiwork. However, these are some of the main types to be aware of.

Watching For Vandalism
Some Wikibookians watch the Recent Changes list to try and monitor for vandalism. Other users monitor the vandalism alerts on IRC. Sometimes users will find vandalism and report them at the Administrative Assistance reading room. A good admin should keep these pages on their watchlist, or find another way to stay alerted about vandalism.

Many types of vandalism will become obvious on the recent changes list, such as page-move vandalism, edit summary vandalism, or page blanking.

Fighting Vandalism
When fighting vandalism, the first task is to block the offending user. The length of the block depends on various factors. Once the user is blocked, you can start reverting the vandal edits.

However, if there are multiple vandals, checkuser help is going to be needed to determine where the vandals are coming from. A checkuser can determine if multiple vandal accounts are coming from the same computer and if so, block all of them at once. Another benefit to checkusers is that when a source for vandalism has been blocked, no new vandal accounts can be created from that computer. Here is a general checklist or playbook to follow when dealing with a vandal:

 * 1) Block the vandal or vandals first.
 * 2) If there are multiple vandals, post a message on WB:AN to get help from a checkuser. If no checkusers are around, the stewards can be contacted at Meta to do the job.
 * 3) If there is a lot of vandalism, request help cleaning it at WB:AN too.
 * 4) Start cleaning the vandalism by reverting all page edits made by the vandal.

If the vandal creates inappropriate pages or uploads inappropriate images as part of the vandalism, they cannot be reverted and need to be deleted. Admins can delete the page, while other users can mark it with.

Open Proxies
A side note to the discussion about vandalism is the discussion about open proxies. An open proxy is an internet IP address which is available for other people to use. The result is that a person can appear to be editing from the open proxy, while concealing their real IP address. WMF policy is that open proxies should be banned for a year. While this isn't always ideal (there are many people who rely on open proxies for regular internet access) it does cut down on vandalism dramatically.

Some admins choose to mass-block open proxies, some admins choose not to bother with it. Technically, an open proxy can be blocked indefinitely, but we don't make people do it if they don't want to.

Vandalism Waves
Some vandals are highly determined, and may use multiple user accounts and multiple IP addresses (typically open proxies) to attack. Sometimes, these attacks can feel overwhelming. Here are some tips for getting through a difficult vandalism attack:

 * Block first, revert second:This way, the vandal won't be creating more messes while you go to clean up the previous messes. Also blocked users can request to be unblocked on their user talk pages. This means that if a person is blocked in error, those blocks can be reversed.
 * Use bot rollback:This advanced technique hides flood vandalism, and the reverts, from Special:RecentChanges.
 * Get Help:Even formidable administrators can use some support when dealing with a tricky vandal. Multiple users and admins should tackle the problem together. Ordinary users can recruit administrator help by posting a request at Administrative Assistance or reporting in IRC. Remember that even non-admins can undo page edits or (for reviewers) revert the page back to a good revision in the page history. The only thing admins are really needed for is blocking vandals and deleting pages.
 * Watch the Recent Changes List:Some users watch the list in real time on IRC, but refreshing the Special:Recentchanges page every couple of minutes is good too. Make sure you keep track of what pages are being targeted, and where the cleanup effort is making progress.
 * Temporarily protect pages:Administrators may need to protect, for a short period of time, pages which normally wouldn't be protected against editing. If a vandal is persistently attacking one page repeatedly, protecting that page will slow the vandal down (or cause the vandal to lose interest all together). If you need to protect a page that gets heavy traffic (such as the reading room or other discussion pages) leave a message about the protection.

Blocking and Unblocking
Blocking users is a necessary weapon in the fight against vandalism. However blocking non-vandals is something that should almost never be done, except in the most extreme circumstances. User blocks can be very disruptive and demoralizing. For that reason, except in cases of obvious vandalism or serial spam should a user be blocked. However, a block is not a one-size-fits-all solution, there are many options that can be set by the blocking admin to affect the way a block is performed.

Block Duration
The duration of a block is the most obvious option that an admin can modify. A user can be blocked for as little as a few minutes, and as long as indefinitely. When deciding a block duration, the admin should check how long similar blocks have been for in the past, and how serious was the offense.

Users who are being disruptive but are acting like a new user (adding "friends of gays" vandalism, or making test edits) should be talked to first. If they don't respond to messages, they can be blocked for a short period. If you block a user temporarily, leave a message on their talk page explaining how long the block was, why they were blocked, and what the expectations are for them to return to regular editing. Some users will just disappear after that, others will learn their lesson and become normal editors here.

Some users are not so harmless and actually make a concerted effort to disrupt the project. These types of users are "vandals", and can be dealt with more suddenly and harshly. These users, who are only interested in disruption and are not experimenting, are not ever going to become normal editors and do not need to be warned or treated kindly. Accounts which have been created for the sole purpose of vandalism can be blocked indefinitely on sight without warning.

Block Reason
There is a drop-down list of the most common blocking reasons. If possible, a reason should be selected from the list when a user is blocked for a common offense. Sometimes, the reason why a user is blocked is not well-suited to the pre-existing explanations, and a custom explanation is in order. When explaining a user block, be as descriptive as you can to show why the user has been blocked. These reasons will appear in the logs so that they can be considered by future admins who might be considering an unblock request.

Block Options
There are three options when blocking users which may have a profound effect on the user. Some of these options are cryptic:

 * Prevent account creation
 * Preventing account creation means that if an IP address is blocked, the user will not be able to create any accounts from that IP until the block expires. This is primarily useful for when a vandal is using a series of sock-puppets from a single source IP address.

 * Automatically block the last IP address used by this user, and any subsequent IPs they try to edit from
 * The second option, often called "autoblock" is used when blocking a username when the IP address is not known. This option blocks that user, blocks the user's IP address, and any additional IP addresses that the user might try to use. This can be a very valuable vandalism fighting tool, especially when sock-puppetry is suspected and no Checkusers are available to run a check. There are two things to note about this kind of block:
 * This may large groups of IP addresses to be blocked automatically, including proxy addresses that are shared by many legitimate users (such as AOL shared IP addresses).
 * Each block will show up individually in the block logs, and to remove the blocks or to modify the block time, each block will have to be examined individually (this can become quite a large number).

 * Prevent user from sending e-mail
 * Blocked users are, by default, enabled to send email to other users. This is a useful aid in discussing a block, or requesting an unblock. However users which are blatant vandals or spammers should have this right revoked to prevent spam or vandalism emails.

Changing Block Parameters
Historically, one had to unblock and re-block a user to change the settings of the block. Recently, this was changed so administrators can change the settings without unblocking. Use the "change block settings" link, or simply use Special:Block/User.

Partial blocks
Historically, the only way to block a user was to prevent them from editing the entire site. With the introduction of partial blocks, it is now possible to block users to, for instance, a particular textbook. This is very useful if the contributor is problematic only in that section of the project (for instance, edit warring) and is otherwise a productive editor. It is also possible to allow full editing but block email access only.

Unblock Requests
A user can request unblock on their user talk page. Administrators should consult the admin who placed the block, except in obvious, uncontroversial cases. An admin may, at their discretion, unblock the user or engage in a discussion with that user about the terms for being unblocked. Caution and discretion are advised, however, because many vandals use bogus unblock requests as an additional form of harassment and vandalism. Users abusing the ability to edit their talk page should have that ability revoked. Previously, this was done by full-protecting the page, but administrators can now adjust the block settings to disallow the blocked user from editing the user talk page. If abuse continues from IPs or new accounts, then protection should be placed in addition to blocking other users/IPs.

CheckUsers are privy to private system logs which are not accessible by the public or administrators. Blocks marked as or "per CU" etc are not to be lifted without consultation with project CheckUsers.

Emergency: Rogue Admin
Possibly the worst-case scenario is a rogue admin or bureaucrat account. It has never happened on Wikibooks, but it has happened on other wikis, notably the English Wikipedia and English Wiktionary, before. Typically, this happens when an administrator's password is discovered by a vandal. It is not unheard of on Wikipedia for a regular admin to become disgruntled with the project and act out negatively as well. The vandal using the administrator's account, or the disgruntled admin, can then go on a rampage blocking ordinary users, deleting pages and unblocking other vandals to destroy the project.

A quick solution would be to block the rogue admin, who can do nothing but block you (admins can no longer unblock themselves). Bureaucrats are not able to demote an administrator, so the only help after that will come from the stewards at Meta. In the event of a rogue admin, the first and most important task is for people to request help from the stewards and/or block the rogue admin if possible. Once the admin has been demoted and blocked, the cleanup effort can begin.

This scenario is a major reason why all Wikibookians, especially those with additional tools, should choose a strong password and (for admins) consider applying two-factor authentication. If a vandal is able to get access to your account, they can cause long term damage not only to the project but to your reputation as well.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Using Wikibooks/Vandalism


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Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. e5/3...c5

=French Defence: Advance Variation= Black's plan is straightforward: They want to eliminate White's pawns on d4/e5 - and after doing so, Black will find it easier to develop the pieces of their kingside to good squares.

White's most common continuation is 4. c3, in order to support their d4-Pawn. If Black follows with 4...cxd4, White can simply replace their d-Pawn with 5. cxd4, and Black's position will be just as cramped as it was before.

There are some rarely played alternative variations for White: Both 4. Nf3 and 4. Qg4 have the idea of (temporarily) sacrificing the d4-pawn in order to get some initiative on the kingside as a compensation. White can also play 4. dxc5, though this helps Black's intention of eliminating White's pawn center.

Theory table
1.e4 e6 2.d4 d5 3.e5 c5


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Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. e5/3...c5/4. c3

=French Defence: Advance Variation= Here, after White's 4. c3, Black usually decides to increase the pressure on White's center pawns (d4/e5) by playing 4...Nc6. Another plausible plan for Black is 4...Qb6, which also increases the pressure on d4, and allows the manoeuver 5...Bd7 followed by 6...Bb5. The idea is to trade Black's 'bad' bishop for White's 'good' bishop.

Theory table
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1.e4 e6 2.d4 d5 3.e5 c5 4.c3


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Hydroculture/Passive/Orchid

Passive hydroponics is one of many methods to grow plants without soil. Growing medium is inert and wicking. It delivers water with fertilizer to the roots by means of capillary action. Substrate contains many small air pockets and can thus deliver oxygen to the roots. The method has been applied to orchids.

Container
Orchids can be planted in any non glass container. The 3-4 drainage holes are placed not at the bottom but 3-5 cm up, at the sides of the pot. The idea is to provide a water reservoir at the bottom of the container from which the medium wicks moisture to the roots.

Popular media
LECA (Lightweight Expanded Clay Aggregate) - expanded / fired clay pellets or clay pebbles, perlite, vermiculite, diatomaceous earth, gravel, charcoal, rockwool, coconut husk chips and their combinations.

Water
Substrate is flushed with tepid water solution when reservoir is nearly empty. Translucent pot may help to see when it is.

Fertilizer
Orchids are fertilized with 1/2 to 1/4 of recommended strength of balanced inorganic fertilizer with every watering. Container is flushed with plain water every month to prevent harmful salt build up.

Conversion from conventional culture to passive hydroponics

 * New medium is washed and then soaked overnight. The plant is then removed from the old pot and substrate is removed from the roots. Rotten or overlong roots are cut / trimmed away. All the roots are thoroughly washed with lukewarm water.

 * Some new medium is placed at the bottom of the new container. The plant is accommodated and more new medium is put around. The pot is gently shaken, more medium is put in, more shaking and so on. Container is flushed with tepid water. The orchid is placed in the shade without fertilizing the following month.

Which orchids can be grown?
Most popular orchids will more or less thrive in hydroponic culture: Paphiopedilum, Phragmipedium, Masdevallia, Phalaenopsis, Cattleya, Cymbidium, Oncidium, Dendrobiums, Epidendrums, Miltoniopsis, Pleurothallid and Zygopetalum.

 * Exceptions would be very big or "thirsty" plants or those whose roots must sometimes completely dry and even those that require dry rest like Dendrobium nobile.

Advantages of Passive Hydroponics
Its simplicity and effectiveness. No guessing about watering and fertilizing, no media decomposition, practically no root rot, healthy plants, fine blooming, no moving parts, low cost, reusable media.
 * In hot and dry environments passive hydroponics is beneficial since the roots stay in a high humidity chamber with some air flow.

Disadvantages

 * The main disadvantage is sometimes need of more frequent watering, especially when plants begin to fill their pots, are big or otherwise demand lots of water. Obviously, a bigger pot / reservoir may help.
 * If the translucent container receive enough light then algae will grow on the outer layer of the potting media. This is mainly an aesthetic concern and not a big problem indoors.
 * If the medium consists of expanded clay pebbles and the plant is newly established then tipping it over may cause the spilling of the medium and plant.
 * Build up of salts from fertilizer and water are not easily removed from clay substrate. Also, excess of chemicals used for pest or disease control is retained.

title: Hydroculture/Passive/Orchid


text:
User talk:Jmak~enwikibooks

I´m Swedish radiologist. I like orchids. --Jmak 09:43, 30 August 2007 (UTC)

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Your account will be renamed
Hello,

The developer team at Wikimedia is making some changes to how accounts work, as part of our on-going efforts to provide new and better tools for our users like cross-wiki notifications. These changes will mean you have the same account name everywhere. This will let us give you new features that will help you edit and discuss better, and allow more flexible user permissions for tools. One of the side-effects of this is that user accounts will now have to be unique across all 900 Wikimedia wikis. See the announcement for more information.

Unfortunately, your account clashes with another account also called Jmak. To make sure that both of you can use all Wikimedia projects in future, we have reserved the name Jmak~enwikibooks that only you will have. If you like it, you don't have to do anything. If you do not like it, you can pick out a different name.

Your account will still work as before, and you will be credited for all your edits made so far, but you will have to use the new account name when you log in.

Sorry for the inconvenience.

Yours, Keegan Peterzell Community Liaison, Wikimedia Foundation 23:20, 17 March 2015 (UTC)

Renamed
 This account has been renamed as part of single-user login finalisation. If you own this account you can |log in using your previous username and password for more information. If you do not like this account's new name, you can choose your own using this form after logging in: . -- Keegan (WMF) (talk) 05:05, 19 April 2015 (UTC)

title: User talk:Jmak~enwikibooks


text:
Computer Systems Engineering/Reliability models

Definition
A system is a combination of elements forming a unitary whole.

Examples

 * River or transportation system
 * System of currency
 * Comprehensive assemblage of facts, principles, and doctrines in a particular field
 * System of marking, numbering, measuring, etc.
 * University of South Carolina – composed of the main campus in Columbia and many branch campuses
 * Computer (our main interest) – includes components: memory, processor, motherboard, disk, printer, wireless adapter, etc.

Every set is not a system. In order to be a system, a set needs a sense of unity, functional relationships between its components, and/or some useful purpose. For example, a random group of items in a room would not be a system unless one of the above conditions are met.

The elements of a system are as follows

 * Components: operating parts for input processing or output
 * Attributes: properties of the components that characterize the system
 * Relationships: links between components and attributes

Components are interrelated and work together toward some purpose, objective, or function. The properties and behavior of each component affect the properties of the system as a whole. For example, the speed of computer memory, the disk access time, and its capacity will all affect the overall speed of a computer. The properties of each component depend on at least one other component. For example, memory performance depends on bus speed (bandwidth). Each subset (or subsystem) of components are related in the same manner, but the system cannot be divided into independent subsets.

Often, a system has a hierarchy of components. A system is made up of components, and those components are made up of smaller components. The lower hierarchical levels are called subsystems. One example is a hard disk drive. The drive is a component of a computer, but it has multiple platters, a read/write head, a buffer, and many more smaller components.

Systems can be classified as

 * Natural and man-made (human-made)
 * Physical and conceptual
 * Static and dynamic
 * Closed and open

Engineering is concerned with the economical use of limited resources in order to benefit people. This is accomplished by approaching a problem with several things in mind. In the domain of systems engineering, it is necessary to define product and system requirements as they relate to true customer needs. For example, designing an email system to meet a customer’s communication needs must be well-defined to meet those needs. Engineering also must address total systems, with all elements, from a life-cycle perspective. The overall hierarchy must be considered, including the interactions between various levels and elements at the same level. An example of this in a computer system is the memory hierarchy, composed of a 2-level cache, main memory, and virtual memory on a hard disk. It is often necessary to organize various related disciplines into one engineering effort in a timely, concurrent manner, such as separate mechanical and electrical aspects of a system. Finally, it is vital to establish a disciplined approach to a process (manage a process to get results). This includes appropriate review, evaluation, and feedback to ensure orderly and efficient progress.

A system’s life cycle is composed of the following

An example of this process in application is as follows. Dictators in third-world countries often want to ride around in fancy cars. However, there is not much support for this preference. Filling stations are not very ubiquitous, and the economy may not support many trained mechanics for automotive repairs. So, from an engineering standpoint, this system would require much more design and money to make it viable.

Summary of Systems Engineering

 * Top-down: look at the system as a whole
 * Life-cycle orientation

1. Design, development production/construction, distribution, operation, maintenance & support, retirement, phaseout, disposal 2. Past emphasis on design & acquisition, with little emphasis on production, operation, maintenance, support & disposal 3. Example: If an old computer goes to a landfill (taking up space and polluting the groundwater), a better design would allow the recovery of gold, lead, and other materials upon disposal. 1. Systems usually require multiple disciplines 2. Example: In the development of a computer game, a company has 3 employees – an artist, a musician, & a programmer.
 * Better definition of system requirements - Trace down customer needs to individual components
 * Interdisciplinary

Definition
“Reliability is the probability of a device performing adequately for the period of time intended under the operating conditions encountered.” – NASA

Math Model of System Reliability
Reliability, R(t), is the probability of a system not failing during the period [0,t].

Experiment
Test a large number of systems.

Hazard function, h(t):

Separate variables and integrate:

Summary F(t) is the failure distribution function R(t) = 1-F(t) is the reliability f(t) is the failure density function h(t) is the hazard function

The difference between f(t), h(t):

At time 2 to 3:

Hazard Function
The shape of the hazard function indicates how an item ages. It has an intuitive interpretation as the amount of risk an item is subject to at a time t:

Increasing Hazard Function This is probably the most likely situation, because items wear out or degrade with time. For example, look at mechanical items that undergo wear or fatigue, such as the rubber getting thinner on car tires over time.

Decreasing Hazard Function In this situation, an item improves; that is, an item is less likely to fail as time passes. For example, some metals “work-harden” through continued use. Also, software may improve as bugs are removed.

Bathtub Shaped Failure Rate This situation describes many natural systems and manufactured goods. It is a composite of 3 effects: *early failures due to defects *late failures due to wear out *accidents at a constant rate

Human Life Characteristics

MTTF = 800 years corresponds to a failure rate of or 5 deaths in a population of 4000 in 1 year

Exponential Reliability Distribution
Recall:

This distribution is the most used reliability model. It is valid for many electronic components over most of their lifetimes, and is the basis for MIL-HDBK-217.

Memoryless Property
Let T = item lifetime (R.V.)

This is the conditional probability that a failure distribution for an item that has survived to time s is identical to a brand new item.

One example of this is a fuse. A fuse fails due to a power surge, but does not weaken or degrade over time. The memoryless property, with its used-as-good-as-new assumption, is restricted in applicability. An exponential distribution is easily misapplied for the sake of simplicity: *statistical techniques are particularly tractable *can add failure rates *field data often allow an estimation of only this one-parameter distribution

C provides a quick check of a data set for exponentiality

Weibull Distribution
Waloddi Weibull, a Swedish physicist, introduced this distribution in 1939. It is a generalization of an exponential distribution suitable for modeling lifetimes having constant, strictly increasing, and strictly decreasing hazard functions.

Note that the Weibull Distribution can match different phases of the bathtub curve.

Procedure: 1.	Collect the failure data. 2.	Get the best fit for the data to a Weibull distribution:

If item is still in the burn-in phase *Improve supplier quality *Burn in the system longer *Be more careful while manufacturing

At GE, light bulbs with as little as a 1% variation in their filaments lead to a 25% shorter lifespan.

If attributed to random failures (accidents) *Make stronger components *Derate – use components at less than the rated value *Use newer technology (i.e. software control, longer-life transistors instead of vacuum tubes, etc.) *Make components less environmentally sensitive (i.e. better packaging) *NPN transistors <  PNP transistors For example, halogen and compact fluorescent bulbs use a different technology to extend life. Further, rating of incandescent long-life light bulbs may proceed as follows:

If item is in the wear-out region *Use stronger, longer-lived components *Use newer technology, etc. *Use a different architecture

Measures of System Reliability
Mean Time To Failure (MTTF) This means that only about 37% of items survive more than 1 MTTF. However, this distribution has a very long tail:

Repairable Systems
Mean Time To Repair (MTTR)

Mean Time Between Failures (MTBF) Note that MTBF and MTTF are often used almost interchangeably by some authors.

Steady State Availability
For example, if a system has only 15 minutes of downtime in a 2-year period, then

Reliability Models
For a series system:

The system works if A works and B works and C works and D works.

For example, if

In terms of time,

Suppose that

Observe that for the constant failure rate (exponential) model, a Weibull distribution can be used: but this is much more difficult.

Redundancy

 * Very simple
 * Very appealing
 * Very deceptive

Component reliability = .9

The system works if either component works or fails if both fail. R = 1-P(fail) = 1-P(first fails & second fails) = 1-P(first fails)P(second fails) = 1-P(.1)(.1)			 note independence = .99

Example: Light Bulbs

Series System:

Parallel System:

Uses of Redundancy
Important in military systems Becoming important in commercial systems Important in high availability systems in which the part being repaired must be shut down
 * To increase reliability, availability
 * To eliminate single points of failure
 * Degradable fault tolerance

Another Example

Probability of system failure = (Probability of A failing) AND (Probability of B failing)

Observe that this is not exponential.

Combined Series-Parallel Systems

Series-Parallel System Reduction
Combine series or parallel component reliabilities to give an equivalent reliability and reduce the system. See the following examples:

1) reduce D, E

2) reduce B, C and I, F

3) reduce II, III

Examine the two different configurations of the following 4-component system with identical components. The component failure rate is:

Moral: We get the greatest gain in reliability by making a system redundant at the lowest level possible. Generally, it is better to make modules redundant than to duplicate the system.

System Design

Make modules redundant in order to achieve reliability goals.

Example: AM Signal Pickup

Series system:

Redundant Design I Series-Parallel at component level:

Functional parameter will change if a component fails – probably.

Redundant Design II Parallel Series

Same functional parameters

Combine Outputs:

Interfaces between parallel subsystems increase complexity of design (which decreases reliability).

Estimating Reliability
The Parts Count reliability model assumes that the system is in series; this model underestimates the reliability of redundant systems. For redundant systems, the Parts Count model is used to estimate the reliability of the series subsystems and interfaces. Reliability is then computed while considering the redundancy structure.

Using our AM Signal Pickup example again:

Ground Mobile environment (GM)

Series Subsystem:

Interface :

System Reliability Estimate:

Simplex System:

Simple Redundant System (ignoring interface problem):

R = .9876

Note: In some cases the interface reliability may dominate the redundant subsystem reliability and determine the overall system reliability. In this case the simplex system may be more reliable than the redundant system.

Non-Series/Parallel System Reduction
Use decomposition:
 * Find the keystone component and partition the system according to whether the keystone component is good or bad.
 * The keystone component binds together the reliability structure of the system.

Law of Total Probability

Example:

Choose A as the keystone component.

If A is good:

Series/Parallel System

If A is bad: Series System

Convenient Notation

Notes:
 * If the “wrong” keystone component is chosen, the component decomposition technique works, but reduction is not as extensive.
 * New keystone components can be repeatedly chosen to further reduce a subsystem.

Parallel Redundancy
This system works as long as 1 module works. M-out-of-N System Reliability

This system works if at least M modules work.

It can tolerate up to N-M failures, so

Voting Systems

The Voter compares the outputs of all N modules and outputs the majority. This is called N Modular Redundancy (NMR). The NMR system will generally have an odd number of modules, so. The system works if (n+1) modules are working (it can have up to n failures), and if the voter is working. Simple Voter

Analog Signal or Numeric Voting

The voter compares input signals (or numeric values) and picks the middle value as its output. Normal operation is as follows:

However, error conditions may arise:

Note: Reliability calculations assume the worst case conditions:
 * All modules fail in the same logical direction
 * There are no compensating failures (i.e. one module becomes stuck at 1, while another is stuck at 0)

Triple Modular Redundancy (TMR)
Example:

Comparison of NMR System Reliabilities Measure time in units of MTTF.

The following figure depicts the reliability of an NMR system for increasing N:

Observe:

Extra hardware increases reliability for the short term but once redundancy is used up there is simply more hardware to fail and reliability decreases quickly.

For a TMR system:

For redundant systems MTTF may not be an appropriate measure of reliability. It is necessary to look at R(t) in relation to mission time.

Cascading TMR

A voter gives a simple point of failure, so a design may triplicate the voter (TMR).

System reliability is determined by 3 parallel modules in the first stage, a voter in the last stage, and a parallel voter-module in the intermediate stage.

NMR Systems
Failed modules accumulate in an NMR system until they become the majority and the system fails. The system life can be extended by purging all of the failed modules. This can be accomplished through Hybrid Redundancy (using spares), or through Adaptive Voting (also called Change Voting). In essence, the failed module(s) must be detected first.

This system has the following attributes:
 * N+S modules (S spares)
 * Disagreement Detector compares the voted output with the module outputs
 * Switch selects the outputs from N modules to give to the voter
 * If a module fails, the Disagreement Detector tells the switch to replace the failed module with a spare one

This configuration is often used with TMR systems. If more than a few spares are switched, the complexity increases to a point where its reliability dominates the system reliability.

N-ary Programming (TMR)
Say we have 3 programmers write code and then vote on the results. In a TMR system, each program could execute on a completely different set of hardware. However, software is labor-intensive and very expensive to produce. N-ary programming significantly increases this cost, does not protect against specification errors, and introduces timing and coordination problems since each of the programs is not identical to the others.

Adaptive Voting
In adaptive voting, the voted output is compared with the module outputs. When a module fails, it is removed along with one other module (this is to keep an odd number of modules). The voter is then changed to select the majority of the remaining modules. This approach can be combined with hybrid redundancy in order to switch good modules back in. Voting (particularly TMR) is used in many fault-tolerant, very-high-reliability computer systems.

Standby Redundancy

 * Operate with A
 * Switch to B when A fails
 * A and B are  not  independent

In general, A and B can be different (i.e. A can be an on-line power source while B can be a generator). It should be noted that B can fail while in its standby mode, or the switch could fail. Examine the following simple case. Assume:

Reliability is as follows:

Recall that the above is the law of total probability.

Therefore,

A sequence of failures forms a process that starts over each time a device fails and a new one is switched in. This is called a renewal process. The time between failures is exponentially distributed,  where X is a random variable denoting the time between failures. Suppose we have n systems as follows:

Recall that for a Poisson process, i)	Events in non-overlapping intervals are independent ii)	P(event in small interval h) = P(no event in h) = iii)	The time between events, X, is exponentially distributed, iv)	The number of events in an interval T,n(T) has the Poisson distribution.

Also recall (for iii) that

Furthermore,

As you might have deduced, this sequence of failures is a Poisson process. Therefore,

For an n component system:

As an overview, compare the following for 2 units:
 * Standby Redundancy:
 * Parallel Redundancy:
 * Simplex System:

 * Standby Redundancy:
 * Parallel Redundancy:
 * Simplex System: R = p


text:
Computer Systems Engineering/Markov models

Markov Models
State diagrams describe the behavior of a system. They describe all possible states of a system (object in UML) and how the state changes as a result of events that reach the system (object).

For example,

It should be noted that Markov Models treat state transitions as a stochastic process.

Let each state be defined by a random variable. The set of random variables {xn} forms a Markov chain if the probability that the next value (state) is xn+1 depends only on the present value (state) xn and not on any previous values. The effect of the entire previous history on future behavior is summed up in the current state. For “discrete time” Markov chains, transitions can only occur at the integers 1,2,3… For “continuous time” Markov chains, transitions can occur at any time. This is important since the past history is specified entirely by the present state, and we cannot also specify how long a process has been in the present state.

Analytically: P(x(tn+1) = xn+1 | x(tn) = xn, x(tn-1) = xn-1, … x(t1) = x1) = P(x(tn+1) = xn+1 | x(tn) = xn) where t1 < t2 < … < tn < tn+1 and x1, x2, …, xn are in some discrete state space.

Consider the discrete time Markov chain:

Observe: the sum of probabilities of leaving each city is 1.

P(xn = j | x1 = i1, x2 = i2, …, xn-1 = in-1) = P(xn = j | xn-1 = in-1).

P(xn = j | xn-1 = in-1) is a one-step transition probability. The probability goes from one state E=i at step n-1 to state E=j at step n.

Let Pij(n) = probability that a system goes from state Ei to state Ej in n-1 steps. Then, is the probability that the system goes from state Ei to state Ek in one step and then goes from state Ek to state Ej in n-1 steps. This is called the Chapman-Kolmogorov equation.

Let P(n) be a matrix. Each entry is Pij(n). If this is a matrix of n-step transition probabilities, then

(called homogenous if this is independent of n)

Define Pij = P(xn = j | xn-1 = i) as the probability that a system goes to the next state Ej given in state Ei. This is called the set of one-step transition probabilities.

Example: P00 = 0, P01 = ¾, P02 = ¼ , P12 = ¾

In matrix form: Let πj = P(xn = j) be the probability that the system is in state j at step n. In our example,

The probability in state j at step n+1 is the probability in state i at step n and goes to state j in one step.

In vector form:

In particular,

For example: Let Π(0) = [0,1,0], starting in Columbia.

After a long time, let

Therefore,

Example:

Observe:

The sum of (1) and (2) is negative (3), so therefore there is a linear dependence between the equations, and thus, there is no unique solution. We also know that π0 + π1 + π2 = 1 (i.e. the system must be in some state). This is called a normalizing condition.

Now solve:

Observe:

This is like flipping a coin, where r = probability of heads and 1-r = probability of tails. So, P(sequence of exactly m heads) = rm (1-r).

How long does a process remain in a state i?
Suppose the process enters state Ei. Then the probability of remaining in state Ei at the next step is Pii. The probability that the process leaves Ei at the next step is 1-Pii. Each time step is independent, so P(system remains in state Ei for exactly m additional time steps given that it has just entered state Ei) = (1-Pii) Piim. In other words, the process stays for m time steps with a probability of m*( Pii Pii Pii …Pii) and leaves the state on the m+1 step.

Geometric distribution
The average time in state Ei is E(time in state E):

Now,, therefore:

Therefore,

In short, the average time a process stays in a state before leaving is the inverse of the probability that it leaves.

Assume a watch state i for k steps and also that the system does not change state. What is the probability that the process does not leave the state on the next step? Let y = the steps to the first departure.

Thus the process is memory less – as we knew.

Continuous Time Markov Chains
P(x(tn+1) = j | x(tn) = in … x(t0) = i0) = P(x(tn+1) = j | x(tn) = in)

Recall that Markov chains are memoryless, so a transition does not depend on how long a process is in a given state.

Let τi = the amount of time in a state i. P(τi > s+t | τi > s) = h(t), which is only a function of t (not s). The function shows how long a process is in a state.

Thus, exponential is the only continuous distribution that satisfies this. Again, if we set s,t = 0, then P(τi >0) = 1, and

Defining the time-dependent transition probability
Now, consider the times s ≤ u ≤ t. To get from state Ei at time s to Ej at time t, a process must pass through some state at time u. Therefore,

This is the Chapman-Kolmogorov equation, and is exactly analogous to the discrete case, but is difficult to use directly.

Recall that

In the Chapman-Kolmogorov equation, substitute t+h for t:

Divide by h, take the limit as h approaches 0, u approaches t:

This is called Kolmogorov’s forward equation.

Also, is Kolmogorov’s backward equation.

If time homogeneous transition probabilities do not depend on initial time t but only on interval h, then qij(t) = qij and qj(t) = qj and = Pij(h), then

State transition probabilities and initial distribution determine all properties of a Markov chain. Observe:

Divide by h, take the limit as h approaches 0, v approaches t:

.

If time homogeneous:

State classifications
Recall that

A state is transient if there is a positive probability that a system will not return to that state. Let Xji = number of visits to state i starting at state j. The expected value of Xji, E(Xji) is

A state i is transient if E(Xji) is finite. This implies that Pji(n) approaches 0 as n approaches infinity.

A state i is recurrent if and only if starting from state j, a process returns to state i with a probability of 1. Also,  is infinite.

Consider the time it takes to reach a state i from state j for a recurrent chain. Let fij(n) = probability of the first visit to j from i in n steps. Let fii(n) = probability of the first return from i to i in n steps. Then, the probability of ever visiting j starting from state i is: State i is recurrent if fii = 1, and transient if fii < 1. If fii = 1, then the mean recurrence time is  A state is called recurrent non-null if  is finite, and recurrent null if   is infinite.

For a recurrent state i,  The period of state i, , is the greatest common divisor of a set of positive integers n such that   A system is aperiodic if  = 1, and periodic if  > 1.

A system is absorbing if and only if Pii = 1.

A Markov chain is irreducible if every state can be reached from every other state in a finite number of steps;  All states of an irreducible Markov chain are of the same type. In an aperiodic Markov chain, all states are aperiodic. If one state is periodic with period d, then all are periodic with the same period d.

Continuous Time Markov Chains
Let = random variable = time in state i. A Markov property means the probability of a transition cannot depend on how long the system has been in a state.

Now for s=0, and  we have

s = t = 0 again requires

Now,

Therefore, the time in state is exponentially distributed with parameter which may depend on the state.

We have seen that  = time in state i and that Let t be a very small increment of time h:

This last equation is the probability that a system leaves a state in the next increment of time h. This can be expressed in a Taylor series:

Number the states 0,1… Let = probability of transition to a higher numbered state (birth). Let = probability of transition to a lower numbered state (death). Only allow transitions to the next higher or lower numbered state (or stay in the same state). This is called a birth-death process.

Let P(n=k,t) = probability of being in state k at time t. Consider how a process can get into state n=k at time t+h.

For births: must have been in state n=k-1 and a birth occurred For deaths: must have been in state n=k+1 and a death occurred If no births and no deaths: must have been in state n=k at time t

Flow Balance
The equilibrium balance equations can be intuitively constructed by inspection from the state transition rate diagram below.

This is similar to the state transition diagram we have already seen. In equilibrium, the rate of flow of probability into a state must equal the rate of flow of probability out of that state. Hence

Solving the Balance Equations
The quantity  is called the “traffic intensity” and is usually denoted by  Recall that the queue will be unstable if jobs arrive faster than they can be served. Thus

As we will see  determines most of the properties of interest in the queue. Note that it is determined by the ratio of the arrival and service rates and not their absolute values. Queues with equal traffic intensities can be expected to have similar properties even though their respective arrival or service rates may be quite different.

The constant P(0) is determined using the normalizing requirement, as shown below:

Note the similarity of the last equation to a geometric distribution:

This is the probability that there are k jobs in the queue and is shown below:

The probability that there is at least 1 job in the queue is This is the utilization of the server, the probability that it is busy.

Number in the Queue
The average number of jobs in the queue is

The sum is evaluated by using the following trick: note that

Now since the sum of a derivative is the derivative of the sum, This is shown below as  increases from 0 to 1:

The probability that there are more than some number r of jobs in the queue is This is shown below for several values of

Note that P(n≥r) decreases geometrically in. Recall also that  is the utilization of the server. Thus for lightly utilized servers (small ) the likelihood that there are many jobs in the queue is small. For heavily utilized servers ( near 1) the likelihood that there are a lot of jobs waiting to be served is much higher.

Let T = the average waiting time, Tn = the waiting time for arrival n with n-1 customers in the system, xi = the service requirement of the ith customer (i = 1,2,…,n), xo = the remaining service for the customer in service, and xn = the service requirement of n+1 customers.


text:
More C++ Idioms/Praise

Praise of the Book

 * "Great and valuable work!" -- Bjarne Stroustrup (February, 2009)

title: More C++ Idioms/Praise


text:
SwisTrack/Version 3/Change log

v.3.0.0 Rev. 175

 * image flip now optional, performance of real-time tracking improved
 * it is now possible to specify whether trajectories can merge (marker-less) or not (marker-based)
 * swistrack can now resize the video

v.3.0.0b Rev. 170

 * core upgrade to Intels OpenCV library 0.99
 * GUI upgraded to WxWidgets 2.6.3
 * Possibility to save BMP files
 * Socket Server added
 * Clients for Matlab, Java, C++ added
 * Coverage image can be refreshed by clicking the right mouse button
 * Improved error handling / user feedback
 * XML description for most of the GUI's features
 * XML data structure for passing parameters within SwisTrack
 * Segmentation routines improved
 * VfW camera (USB) support added
 * Panels added for most classes

v.2.4.1 (05.08.2005)

 * core upgraded to Intels OpenCV library 0.96

v.2.4.0 (15.06.2005)

 * data association can be overrun manually at any time
 * single step mode
 * bugix: GUI reflects tracking state properly after tracking has terminated

v.2.3.1 (31.05.2005)

 * bugfix: number of colorplanes taken into account when using background subtraction

v.2.3.0 (09.05.2005)

 * tracking improved drastically. Merged trajectories are split if there are better candidates.

v.2.2.2 (08.05.2005)

 * color image segmentation improved (segmentation on green plane)

v.2.2.1 (02.05.2005)

 * bugfix: background estimation could not be selected
 * bugfix: the program crashes on shutdown when working with the IEEE Camera

v.2.2.0 (14.04.2005)

 * Mask mode added. Binary images supplied by the users supply contours and the objects within this contours gets counted

v.2.1.1

 * Visualisation 'Coverage added'

v.2.1.0

 * Support for IEEE1394 (Firewire) Cameras added
 * Background estimation for segmentation (instead of static background) added

v.2.0.1

 * Possibility to export video data of object trajectories
 * HTML help added

v.2.0 (28.07.2004)

 * Windows look'n'feel compliant GUI
 * Integration of Tracking/Calibration/Data Storage under a single platform

title: SwisTrack/Version 3/Change log


text:
User:Kanjy

My user accounts of:
 * w:ja:User:Kanjy ( talk ) - Japanese Wikipedia, my home wiki
 * ja:User:Kanjy (talk) - Japanese Wikibooks, semi-home
 * m:User:Kanjy (talk) - Meta
 * Kanjy - my global account
 * English wikis: Wikipedia - Wiktionary - Wikibooks - Wikiquote - Wikinews - Wikisource - Wikiversity

de:Benutzer:Kanjy es:Usuario:Kanjy fr:Utilisateur:Kanjy it:Utente:Kanjy ja:利用者:Kanjy pt:Usuário:Kanjy zh:User:Kanjy

title: User:Kanjy


text:
User:James.sutherland

=James Sutherland=

On:Wikipedia

Location
Ottawa, Canada

Profession
Software Engineer

Currently working for Oracle on there TopLink product.

Also working with Eclipse on their EclipseLink product.

title: User:James.sutherland


text:
Gothic/2/Vocab

Here is your vocabulary for this chapter. Note that there will be other vocabulary in the Pronouns and Articles sections that will be integrated within the tutorial.:

Verbs
Try to memorize them in the Gothic alphabet based on the Latin transcription. In further lessons when these are used, they will not use distinguished Latin characters and you must be able to know how to truly pronounce them. When to use which version of the word "the" will be covered in another page within this chapter.

title: Gothic/2/Vocab


text:
XML - Managing Data Exchange/Basic data structures

Introduction
In reviewing the four central problems in data management, (capture, storage, retrieval, and exchange) the typical user of XML encounters recurring fundamental structural patterns that apply to all sorts of data throughout the storage and exchange phases. These patterns recur consistently because their use transcends the particular contexts in which the underlying data are processed. We call these patterns "data structures" (or datatypes).

In this section, we discuss a few of the most fundamental "basic data structures" and explain why they are useful, as well as how to work with them using XML.

We start our introduction with a simple example. Consider an ordinary grocery shopping list for a single-person household.

Introductory Shopping List Example:

Andy's shopping list: * eggs * cough syrup(pick up for granny) * orange juice * bread * laundry detergent **don't forget this**

When analyzing aspects of the information contained in this shopping list, we can make some basic generalizations:

 * Portability: the shopping list can be represented and transferred easily. If necessary, it could be stored in a database and processed by custom-designed software, but it could just as easily be written on a scrap of paper;
 * Comprehensibility: the shopping list is readily understood by its intended audience (in this instance, the sole person who wrote the list) and therefore needs no additional information or structure in order to be immediately usable;
 * Adaptability: if any changes become necessary (such as additions or removals to the list) there is an existing and well-known methodology for accomplishing this (e.g., in the case of a handwritten list, simply write down new entries or cross out unwanted entries).

The fundamental concept of basic data structures
Given that we have the previous example for background, we can now introduce the fundamental concept of "basic data structures".

Basic data structures defined
Now that we have introduced our concept of data structures, we can start with some concrete definitions, and then review those definitions in the context of our shopping list example.

Overview of "core" data structures
The following terms define some "core" data structures that we use throughout this chapter. This list is ordered in ascending degrees of complexity:
 * SimpleBoolean: Any value capable of being expressed as either "True" or "False".
 * SimpleString: A contiguous sequence of characters, including both alphanumeric and non-alphanumeric.
 * SimpleSequence: An enumeration of items generally accessible by numeric indexing.
 * Name-value pair: An arbitrary singular name attached to a singular value.
 * SimpleDictionary: An enumeration of items generally accessible by alphanumeric indexing.
 * SimpleTable: An ordered arrangement of columns and rows. A SimpleTable can be classified as a "composite" data structure (e.g., SimpleSequence where each item in the sequence is a single SimpleDictionary).

An important point to remember while reviewing these "core" data structures is that they are elemental and complementary. That is, the core structures, when used in combination, can form even more complex structures. Once the reader comes to understand this fact, it will become apparent that there is no conceivable application or data specification that cannot be wholly described in XML using nothing more than these "core" data structures.

Now review the "Introductory Shopping List Example" above. When we compare it with the "core" data structures that we've just defined, we can make some fairly straightforward observations:

 * The entire shopping list cannot be represented using a SimpleBoolean data structure, because the information is more complex than either "True" or "False".
 * The entire shopping list can be represented using a SimpleString.
 * There may be reasons why we would not want to use a SimpleString to represent the entire shopping list. For example, we might want to transfer the list into a database or other software application and then be able to sort, query, duplicate or otherwise process individual items on the list. Treating the entire list as a SimpleString would therefore complicate our processing requirements.

SimpleString
Different ways to represent a SimpleString in XML:

SimpleSequence
Different ways to represent a SimpleSequence in XML:

Side-by-side examples
SimpleTable (XML_Elem):

SimpleTable (XML_Attr):

SimpleTable (XML_Mixed):

Basic data structures in programming
To further illustrate how basic data structures apply in many different contexts, some of the basic data structures enumerated previously are examined and compared here in the context of computer programming.

For the first part of the comparison, we examine the generic terminology against that used commonly in programming languages:

 * SimpleBoolean: is commonly called a  and can usually take the values   or ,   or  , or other values, depending on the language.
 * SimpleString: commonly called a  or.
 * SimpleSequence: numerically indexed variables in programming are commonly represented with an.
 * Name-value pair: (explained in more detail below)
 * SimpleDictionary: these are commonly represented with a, or an.
 * SimpleTable: (explained in more detail below)

Technical considerations
Now that we've introduced and discussed specific examples of the basic data structures, there are a few technical considerations that apply to all of the data structures, and are particularly important to those who may be responsible for implementing and designing XML schemas to deal with specific implementation scenarios.

 * Exact terminology depends on context: Although the "basic" structures described here apply to many different scenarios, the terms used to describe them can overlap or conflict. For example, the term "SimpleSequence" as used here closely coincides with what is called an "array" in many programming languages. Similarly, the term "SimpleDictionary" is shorthand for what some programming languages call an "associative array". Although this close correlation is intentional, one must always remember that the specific nuances of an application or programming language will require additional attention. Sometimes minor conflicts or discrepancies arise when one digs into the details for any specific data structure in any given project or technology.

 * Basic structures are flexible concepts: Structures can be defined in terms of one another, and some structures can be applied recursively. For example, one could easily define a SimpleSequence using a SimpleString along with some basic assumptions. (e.g., a SimpleSequence is a string of alphanumeric characters where each item in the sequence is separated by one or more whitespace characters: "eggs bread butter milk").

 * Abstract structures tend to hide tricky details: For example, the term "SimpleString" describes the abstract notion of a sequence of characters (e.g., "ISBN 0-596-00327-7"). The abstract notion is fairly intuitive and uncomplicated. Nevertheless, the precise notation used to implement that abstract notion, and represent it in real-live working code is a different matter entirely. Different programming languages and different environments may use different conventions for representing the same "string". Because of this variability, one can also recognize that the abstract notion of a "SimpleString" in XML is also subject to differing representations, based on the needs of any given project.

Notes and references
Programmation XML/Structure d'un document XML


text:
Contemporary Educational Psychology/Chapter 10: Teacher-made Assessment Strategies/Assessment That Enhances Motivation

Assessment that Enhances Motivation and Student Confidence
Studies on testing and learning conducted more than 20 years ago demonstrated that tests promote learning and that more frequent tests are more effective than less frequent tests (Dempster & Perkins, 1993). Frequent smaller tests encourage continuous effort rather than last minute cramming and may also reduce test anxiety because the consequences of errors are reduced. College students report preferring more frequent testing than infrequent testing (Bangert-Downs, Kulik, Kulik, 1991). More recent research indicates that teachers’ assessment purpose and beliefs, the type of assessment selected, and the feedback given contributes to the assessment climate in the classroom which influences students’ confidence and motivation. The use of self-assessment is also important in establishing a positive assessment climate.

Teachers’ Purposes and Beliefs
Student motivation can be enhanced when the purpose of assessment is promoting student learning and this is clearly communicated to students by what teachers say and do (Harlen, 2006). This approach to assessment is associated with what the psychologist Carol Dweck (2000) calls an incremental view of ability or intelligence. An incremental view assumes that ability increases whenever an individual learns more. This means that effort is valued because effort leads to knowing more and therefore having more ability. Individuals with an incremental view also ask for help when needed and respond well to constructive feedback as the primary goal is increased learning and mastery. In contrast, a fixed view of ability assumes that some people have more ability than others and nothing much can be done to change that. Individuals with a fixed view of ability often view effort in opposition to ability (“smart people don’t have to study”) and so do not try as hard, and are less likely to ask for help as that indicates that they are not smart. While there are individual differences in students’ beliefs about their views of intelligence, teachers’ beliefs and classroom practices influence students’ perceptions and behaviors.

Teachers with an incremental view of intelligence communicate to students that the goal of learning is mastering the material and figuring things out. Assessment is used by these teachers to understand what students know so they can decide whether to move to the next topic, re-teach the entire class, or provide remediation for a few students. Assessment also helps students’ understand their own learning and demonstrate their competence. Teachers with these views say things like, “We are going to practice over and over again. That’s how you get good. And you’re going to make mistakes. That’s how you learn.” (Patrick, Anderman, Ryan, Edelin, Midgley, 2001, p. 45).

In contrast, teachers with a fixed view of ability are more likely to believe that the goal of learning is doing well on tests especially outperforming others. These teachers are more likely to say things that imply fixed abilities e.g., “This test will determine what your math abilities are,” or that stress the importance of interpersonal competition, “We will have speech competition and the top person will compete against all the other district schools and last year the winner got a big award and their photo in the paper.” When teachers stress interpersonal competition some students may be motivated but there can only a few winners so there are many more students who know they have no change of winning. Another problem with interpersonal competition in assessment is that the focus can become winning rather than understanding the material.

Teachers who communicate to their students that ability is incremental and that the goal of assessment is promoting learning rather that ranking students, awarding prizes to those who did very well, or catching those who did not pay attention, are likely to enhance students’ motivation.

Choosing Assessments
The choice of assessment task also influence students’ motivation and confidence.
 * Assessments that have clear criteria that students understand and can meet rather than assessments that pit students against each other in interpersonal competition enhances motivation (Black & Wiliam, 2006). This is consistent with the point we made in the previous section about the importance of focusing on enhancing learning for all students rather than ranking students.
 * Meaningful assessment tasks enhance student motivation. Students often want to know why they have to do something and teachers need to provide meaningful answers. For example, a teacher might say, “You need to be able to calculate the area of a rectangle because if you want new carpet you need to know how much carpet is needed and how much it would cost.” Well designed performance tasks are often more meaningful to students than selected response tests so students will work harder to prepare for them.
 * Providing choices of assessment tasks can enhance student sense of autonomy and motivation according to self determination theory (see Chapter 6). Kym, the 6th grade teacher whose story began this chapter, reports that giving students choices was very helpful. Another middle school social studies teacher Aaron, gives his students a choice of performance tasks at the end of the unit on the Bill of Rights. Students have to demonstrate specified key ideas but can do that by making up a board game, presenting a brief play, composing a rap song etc. Aaron reports that students work much harder on this performance assessment which allows them to use their strengths than previously when he did not provide any choices and gave a more traditional assignment. Measurement experts caution that a danger of giving choices is that the assessment tasks are no longer equivalent and so the reliability of scoring is reduced so it is particularly important to use well designed scoring rubrics. Fourth, assessment tasks should be challenging but achievable with reasonable effort (Elliott, McGregor & Thrash, 2004). This is often hard for beginning teachers to do who may give assessment tasks that are too easy or too hard because they have to learn to match their assessment to the skills of their students.

Providing Feedback
When the goal is assessment for learning, providing constructive feedback that helps students know what they do and do not understand as well as encouraging them to learn from their errors is fundamental. Effective feedback should be given as soon as possible, as the longer the delay between students’ work and feedback the longer students will continue to have some misconceptions. Also, delays reduce the relationship between students’ performance and the feedback, as students can forget what they were thinking during the assessment. Effective feedback should also inform students clearly what they did well and what needs modification. General comments just as “good work- A”, or “needs improvement” do not help students understand how to improve their learning. Giving feedback to students using well designed scoring rubrics helps clearly communicate strengths and weaknesses. Obviously grades are often necessary, but teachers can minimize the focus by placing the grade after the comments or on the last page of a paper. It can also be helpful to allow students to keep their grades private making sure when returning assignments that the grade is not prominent (e.g., by not using red ink on the top page) and by never asking students to read their scores aloud in class. Some student choose to share their grades, but that should be their decision, not their teacher's.

When grading, teachers often become angry at the mistakes that student make. It is easy for teachers to think something like, “With all the effort I put into teaching and this student couldn’t even be bothered to follow the directions or spell check!” Many experienced teachers believe that communicating their anger is not helpful, so rather than saying, “How dare you turn in such shoddy work,” they rephrase it as, “I am disappointed that your work on this assignment does not meet the standards set” (Sutton, 2003). Research evidence also suggests that comments such as “You are so smart” for a high quality performance can be counterproductive. This is surprising to many teachers but if students are told they are smart when the produce a good product, then if they do poorly on the next assignment the conclusion must be they are “not smart.” More effective feedback focuses on positive aspects of the task (not the person), as well as strategies, and effort. The focus of the feedback should relate to the criteria set by the teacher and how improvements can be made.

When the teacher and student are from different racial/ethnic backgrounds providing feedback that enhances motivation and confidence but also includes criticism can be particularly challenging because the students of color have historical reasons to distrust negative comments from a White teacher. Research by Cohen Steele, Ross (1999) indicates that “wise” feedback from teachers needs three components: positive comments, criticisms, and an assurance that the teacher believes the student can reach higher standards.

Self and Peer Assessment
In order to reach a learning goal, students need to understand the meaning of the goal, the steps necessary to achieve a goal, and if they are making satisfactory progress towards that goal (Sadler, 1989). This involves self assessment and recent research has demonstrated that well designed self assessment can enhance student learning and motivation. For self assessment to be effective, students need explicit criteria such as those in an analytical scoring rubric. These criteria are either provided by the teacher or developed by the teacher in collaboration with students. Because students seem to find it easier to understand criteria for assessment tasks if they can examine other students’ work along side their own, self assessment often involves peer assessment. An example of a strategy used by teachers involves asking students to use “traffic lights” to indicate of their confidence in their assignment or homework. Red indicates that were unsure of their success, orange that they were partially unsure, and green that they were confident of their success. The students who labeled their own work as orange and green worked in mixed groups to evaluate their own work while the teacher worked with the students who had chosen red.

If self and peer assessment is used it is particularly important that the teachers establish a classroom culture for assessment that is based on incremental views of ability and learning goals. If the classroom atmosphere focuses on interpersonal competition, students have incentives in self and peer assessment to inflate their own evaluations (and perhaps those of their friends) because there are limited rewards for good work.

(back to Chapter 10...)


text:
Contemporary Educational Psychology/Chapter 10: Teacher-Made Assessment Strategies/Adjusting Instruction re Assessment

Adjusting Instruction Based on Assessment
Using assessment information to adjust instruction is fundamental to the concept of assessment for learning. Teachers make these adjustments “in the moment” during classroom instruction as well as during reflection and planning periods. Teachers use the information they gain from questioning and observation to adjust their teaching during classroom instruction. If students cannot answer a question, the teacher may need to rephrase the question, probe understanding of prior knowledge, or change the way the current idea is being considered. It is important for teachers to learn to identify when only one or two students are struggling with the concept so need individual help, and when a large proportion of the class is struggling so whole group intervention is needed.

After the class is over, effective teachers spend time analyzing how well the lessons went, what students did and did not seem to understand, and what needs to be done the next day. Evaluation of student work also provides important information for teachers. If many students are confused about a similar concept the teacher needs to re-teach it and consider new ways of helping students understand the topic. If the majority of students complete the tasks very quickly and well, the teacher might decide that the assessment was not challenging enough. Sometimes teachers become dissatisfied with the kinds of assessments they have assigned when they are grading – perhaps because they realize there was too much emphasis on lower level learning, that the directions were not clear enough, or the scoring rubric needed modification. Teachers who believe that assessment data provides information about their own teaching and that they can find ways to influence student learning have high teacher efficacy or beliefs that they can make a difference in students’ lives. In contrast, teachers who think that student performance is mostly due to fixed student characteristics or the homes they come from (e.g., “no wonder she did so poorly considering what her home life is like”) have low teacher efficacy (Tschannen-Moran, Woolfolk Hoy, & Hoy, 1998).

Communication with Parents and Guardians
Clear communication with parents about classroom assessment is important – but often difficult for beginning teachers. The same skills that are needed to communicate effectively with students are also needed when communicating with parents and guardians. Teachers need to be able to explain to parents the purpose of the assessment, why they selected this assessment technique, and what the criteria for success are. Some teachers send home newsletters monthly or at the beginning of a major assessment task explaining the purpose and nature of the task, any additional support that is needed (e.g. materials, library visits), and due dates. Some parents will not be familiar with performance assessments or the use of self and peer assessment so teachers need to take time to explain these approaches carefully.

Many school districts now communicate though websites that have mixtures of public information available to all parents in the class (e.g. curriculum and assessment details) as well information restricted to the parents or guardians of specific students (e.g., the attendance and grades). Teachers report this is helpful as parents have access to their child’s performance immediately and when necessary, can talk to their child and teacher quickly.

The recommendations we provided above on the type of feedback that should be given to students also apply when talking to parents. That is, the focus should be on students’ performance on the task, what was done well and what needs work, rather than general comments about how “smart” or “weak” the child is. If possible, comments should focus on strategies that the child uses well or needs to improve (e.g. reading test questions carefully, organization in a large project). When the teacher is white and the student or parents are minority trust can be an issue so using “wise” feedback when talking to parents may help.

Action Research: Studying yourself and your students
Assessment for learning emphasizes devising and conducting assessment data in order to improve teaching and learning and so is related to action research (also called teacher research). In Chapter 1, we described action research as studies conducted by teachers of their own students or their own work. Action research can lead to decisions that improve a teacher’s own teaching or the teaching of colleagues. Kym, the teacher we described at the beginning of this chapter, conducted action research in her own classroom as she identified a problem of poor student motivation and achievement, investigated solutions during the course on motivation, tried new approaches, and observed the resulting actions. In recent years, action research has become increasingly valued by educators as a way of learning about and improving their own work. It is so important, in fact, that we offer a more complete discussion of it at the end of this book, in Chapter 13.

Cycles of Planning, Acting and Reflecting
Action research is usually described as a cyclical process with the following stages (Mertler, 2006).

 * Planning Stage Planning has three components. First, it involves identifying and defining a problem. Problems sometimes start with some ill defined unease or feeling that something is wrong and it can take time to identify the problem clearly so that it becomes a researchable question. The next step, is reviewing the related literature and this may occur within a class or workshop that the teachers are attending. Teachers may also explore the literature on their own or in teacher study groups. The third step is developing a research plan. The research plan includes what kind of data will be collected (e.g. student test scores, observation of one or more students, as well as how and when it will be collected (e.g. from files, in collaboration with colleagues, in Spring or Fall semester).
 * Acting Stage During this stage the teacher is collecting and analyzing data. The data collected and the analyses do not need to be complex because action research, to be effective, has to be manageable.
 * Developing an Action Plan In this stage the teacher develops a plan to make changes and implements the changes. This is the action component of action research and it is important that teachers document their actions carefully so that they can communicate them to others.
 * Communicating and Reflection An important component of all research is communicating information. Results can be shared with colleagues in the school or district, in an action research class at the local college, at conferences, or in journals for teachers. Action research can also involve students as active participants and if this is the case, communication may include students and parents. Communicating with others helps refine ideas and so typically aids in reflection. During reflection teachers/researchers ask such questions as, “What did I learn?” “What should I have done differently?” “What should I do next?” Questions such as these often lead to a new cycle of action research beginning with planning and then moving to the other steps.

Ethical issues—privacy and voluntary consent
Teachers are accustomed to collecting students’ test scores, data about performances, and descriptions of behaviors as an essential component of teaching. However, if teachers are conducting action research and they plan to collect data that will be shared outside the school community then permission from parents (or guardians) and students must be obtained in order to protect the privacy of students and their families. Typically permission is obtained by an informed consent form that summarizes the research, describes the data that will be collected, indicates that participation is voluntary, and provides a guarantee of confidentiality or anonymity (Hubbard & Power, 2005). Many large school districts have procedures for establishing informed consent as well as person in the central office who is responsible for the district guidelines and specific application process. If the action research is supported in some way by a college of university (e.g., through a class) then informed consent procedures of that institution must be followed.

One common area of confusion for teachers is the voluntary nature of student participation in research. If the data being collected are for a research study, students can choose not to participate. This is contrary to much regular classroom instruction where teachers tell students they have to do the work or complete the tasks.

(back to Chapter 10...)


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Contemporary Educational Psychology/Chapter 10: Teacher-made Assessment Strategies/Grading and Reporting

Assigning students grades is an important component of teaching and many school districts issue progress reports, interim reports, or mid term grades as well as final semester grades. Traditionally these reports were printed on paper and sent home with students or mailed to students’ homes. Increasingly, school districts are using web-based grade management systems that allow parents to access their child’s grades on each individual assessment as well as the progress reports and final grades.

Grading can be frustrating for teachers because it is always a compromise among many factors. In addition, report cards typically summarize in brief format a variety of assessments and so cannot provide much diagnostic information about students’ strengths and weaknesses. This means that report cards focus more on assessment of learning than assessment for learning. There are a number of decisions that have to be made when assigning students’ grades, and schools often have detailed policies that teachers have to follow. Most of the policies relate to several key questions about assessment, which we consider below.

How are various assignments and assessments weighted?
Students typically do a variety of assignments during a grading period such as homework, quizzes, performance assessments, etc. Teachers have to decide - preferably before the grading period begins - how each assignment will be weighted. For example, a 6th grade math teacher may decide to weight the grades in the following manner:

 * Weekly quizzes		35%
 * Homework 			15%
 * Performance Assessment 	30%
 * Class participation 		20%

Deciding how to weight assignments should be done carefully, since it communicates to students and parents what teachers believe is important, and also may be used to decide how much effort students will exert (e.g., “If homework is only worth 5% it is not worth completing twice a week”).

Should social skills or effort be included? Elementary school teachers are more likely than middle or high school teachers to include some social skills into report cards (Popham, 2005). These may be included as separate criteria in the report card or weighted into the grade for that subject. For example, the grade for mathematics may include an assessment of group cooperation or self regulation during mathematics lessons. Some schools and teachers endorse including social skills arguing that developing such skills is important for young students and that students need to learn to work with others and manage their own behaviors in order to be successful. Others believe that grades in subject areas should be based on the cognitive performances – and that if assessments of social skills are made they should be clearly separated from the subject grade on the report card. Obviously, clear criteria such as those contained in analytical scoring rubrics should be used if social skills are graded.

Teachers often find it difficult to decide whether effort and improvement should be included as a component of grades. One approach is for teachers to ask students to submit drafts of an assignment and make improvements based on the feedback they received. The grade for the assignment may include some combination of the score for the drafts, the final version, and the amount of improvement the students made based on the feedback provided. A more controversial approach is basing grades on effort when students try really hard day after day but still cannot complete their assignments well. These students could have identified special needs or be recent immigrants and have limited English skills. Some school districts have guidelines for handling such cases. One disadvantage of using improvement as a component of grades is that the most competent students in class may do very well initially and have little room for improvement – unless teachers are skilled at providing additional assignments that will help challenge these students.

Teachers often use “hodgepodge grading,” i.e., a combination of achievement, effort, growth, attitude or class conduct, homework, and class participation. A survey of over 8500 middle and high school students in Virginia  supported the hodgepodge practices commonly used by their teachers (Cross & Frary, 1999).

How should grades be calculated?
Two options are commonly used: absolute gradin and relative grading. In absolute grading evaluations are assigned based on criteria the teacher has devised in advance. If an English teacher has established a level of proficiency needed to obtain an A and no student meets that level then no A’s will be given. Alternatively if every student meets the established level then all the students will get A’s. Absolute grading systems may use letter grades or pass/fail.

In relative grading the teacher ranks the performances of students from worst to best (or best to worst) and those at the top get high grades, those in the middle moderate grades, and those at the bottom low grades. This is often described as “grading on the curve” and can be useful to compensate for an examination or assignment that students find much easier or harder than the teacher expected. However, relative grading can be unfair to students because the comparisons are typically within one class, so an A in one class may not represent the level of performance of an A in another class. Relative grading systems may discourage students from helping each other improve as students are in competition for limited rewards. In fact, Bishop (1999) argues that grading on the curve gives students a personal interest in persuading each other not to study as a serious student makes it more difficult for others to get good grades.

What kinds of grade descriptions should be used?
Traditionally a letter-grade system is used (e.g. A, B, C, D, F ) for each subject. The advantages of these grade descriptions are they are convenient, simple, and can be averaged easily. However, they do not indicate what objectives the student has or has not met nor students’ specific strengths and weaknesses. Elementary schools often use a pass-fail (or satisfactory-unsatisfactory) system and some high schools and colleges do as well. Pass-fail systems in high school and college allow students to explore new areas and take risks on subjects that they may have limited preparation for, or is not part of their major. While a pass-fail system is easy to use, it offers even less information about students’ level of learning.

A pass-fail system is also used in classes that are taught under a mastery-learning approach, in which students are expected to demonstrate mastery on all the objectives in order to receive course credit. Under these conditions, it is clear that a pass means that the student has demonstrated mastery of all the objectives.

Some schools have implemented a checklist of the objectives in subject areas to replace the traditional letter grade system, and students are rated on each objective using descriptors such as Proficient, Partially Proficient, and Needs Improvement. For example, the checklist for students in a 4th grade class in California may include the four types of writing that are required by the English language state content standards:

 * Writing narratives
 * Writing responses to literature
 * Writing information reports
 * Writing summaries

The advantages of this approach are that it communicates students’ strengths and weaknesses clearly, and reminds the students and parents the objectives of the school. However, if too many objectives are included then the lists can become so long that they are difficult to understand.

(back to Chapter 10...)


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