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Electricity generation

Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery (transmission, distribution, etc.) to end users or its storage, using for example, the pumped-storage method.

Consumable electricity is not freely available in nature, so it must be "produced", transforming other forms of energy to electricity. Production is carried out in power stations, also called "power plants". Electricity is most often generated at a power plant by electromechanical generators, primarily driven by heat engines fueled by combustion or nuclear fission, but also by other means such as the kinetic energy of flowing water and wind. Other energy sources include solar photovoltaics and geothermal power. There are exotic and speculative methods to recover energy, such as proposed fusion reactor designs which aim to directly extract energy from intense magnetic fields generated by fast-moving charged particles generated by the fusion reaction (see magnetohydrodynamics).

Phasing out coal-fired power stations and eventually gas-fired power stations, or, if practical, capturing their greenhouse gas emissions, is an important part of the energy transformation required to limit climate change. Vastly more solar power and wind power is forecast to be required, with electricity demand increasing strongly with further electrification of transport, homes and industry. However, in 2023, it was reported that the global electricity supply was approaching peak CO2 emissions thanks to the growth of solar and wind power.

History

The fundamental principles of electricity generation were discovered in the 1820s and early 1830s by British scientist Michael Faraday. His method, still used today, is for electricity to be generated by the movement of a loop of wire, or Faraday disc, between the poles of a magnet. Central power stations became economically practical with the development of alternating current (AC) power transmission, using power transformers to transmit power at high voltage and with low loss.

Commercial electricity production started with the coupling of the dynamo to the hydraulic turbine. The mechanical production of electric power began the Second Industrial Revolution and made possible several inventions using electricity, with the major contributors being Thomas Alva Edison and Nikola Tesla. Previously the only way to produce electricity was by chemical reactions or using battery cells, and the only practical use of electricity was for the telegraph.

Electricity generation at central power stations started in 1882, when a steam engine driving a dynamo at Pearl Street Station produced a DC current that powered public lighting on Pearl Street, New York. The new technology was quickly adopted by many cities around the world, which adapted their gas-fueled street lights to electric power. Soon after electric lights would be used in public buildings, in businesses, and to power public transport, such as trams and trains.

The first power plants used water power or coal. Today a variety of energy sources are used, such as coal, nuclear, natural gas, hydroelectric, wind, and oil, as well as solar energy, tidal power, and geothermal sources.

In the 1880s the popularity of electricity grew massively with the introduction of the Incandescent light bulb. Although there are 22 recognised inventors of the light bulb prior to Joseph Swan and Thomas Edison, Edison and Swan's invention became by far the most successful and popular of all. During the early years of the 19th century, massive jumps in electrical sciences were made. And by the later 19th century the advancement of electrical technology and engineering led to electricity being part of everyday life. With the introduction of many electrical inventions and their implementation into everyday life, the demand for electricity within homes grew dramatically. With this increase in demand, the potential for profit was seen by many entrepreneurs who began investing into electrical systems to eventually create the first electricity public utilities. This process in history is often described as electrification.

The earliest distribution of electricity came from companies operating independently of one another. A consumer would purchase electricity from a producer, and the producer would distribute it through their own power grid. As technology improved so did the productivity and efficiency of its generation. Inventions such as the steam turbine had a massive impact on the efficiency of electrical generation but also the economics of generation as well. This conversion of heat energy into mechanical work was similar to that of steam engines, however at a significantly larger scale and far more productively. The improvements of these large-scale generation plants were critical to the process of centralised generation as they would become vital to the entire power system that we now use today.

Throughout the middle of the 20th century many utilities began merging their distribution networks due to economic and efficiency benefits. Along with the invention of long-distance power transmission, the coordination of power plants began to form. This system was then secured by regional system operators to ensure stability and reliability. The electrification of homes began in Northern Europe and in the Northern America in the 1920s in large cities and urban areas. It was not until the 1930s that rural areas saw the large-scale establishment of electrification.

Methods of generation
Several fundamental methods exist to convert other forms of energy into electrical energy. Utility-scale generation is achieved by rotating electric generators or by photovoltaic systems. A small proportion of electric power distributed by utilities is provided by batteries. Other forms of electricity generation used in niche applications include the triboelectric effect, the piezoelectric effect, the thermoelectric effect, and betavoltaics.

Generators
Electric generators transform kinetic energy into electricity. This is the most used form for generating electricity and is based on Faraday's law. It can be seen experimentally by rotating a magnet within closed loops of conducting material, e.g. copper wire. Almost all commercial electrical generation is done using electromagnetic induction, in which mechanical energy forces a generator to rotate.

Electrochemistry

Electrochemistry is the direct transformation of chemical energy into electricity, as in a battery. Electrochemical electricity generation is important in portable and mobile applications. Currently, most electrochemical power comes from batteries. Primary cells, such as the common zinc–carbon batteries, act as power sources directly, but secondary cells (i.e. rechargeable batteries) are used for storage systems rather than primary generation systems. Open electrochemical systems, known as fuel cells, can be used to extract power either from natural fuels or from synthesized fuels. Osmotic power is a possibility at places where salt and fresh water merge.

Photovoltaic effect
The photovoltaic effect is the transformation of light into electrical energy, as in solar cells. Photovoltaic panels convert sunlight directly to DC electricity. Power inverters can then convert that to AC electricity if needed. Although sunlight is free and abundant, solar power electricity is still usually more expensive to produce than large-scale mechanically generated power due to the cost of the panels. Low-efficiency silicon solar cells have been decreasing in cost and multijunction cells with close to 30% conversion efficiency are now commercially available. Over 40% efficiency has been demonstrated in experimental systems.

Until recently, photovoltaics were most commonly used in remote sites where there is no access to a commercial power grid, or as a supplemental electricity source for individual homes and businesses. Recent advances in manufacturing efficiency and photovoltaic technology, combined with subsidies driven by environmental concerns, have dramatically accelerated the deployment of solar panels. Installed capacity is growing by around 20% per year led by increases in Germany, Japan, United States, China, and India.

Economics
The selection of electricity production modes and their economic viability varies in accordance with demand and region. The economics vary considerably around the world, resulting in widespread residential selling prices. Hydroelectric plants, nuclear power plants, thermal power plants and renewable sources have their own pros and cons, and selection is based upon the local power requirement and the fluctuations in demand.

All power grids have varying loads on them. The daily minimum is the base load, often supplied by plants which run continuously. Nuclear, coal, oil, gas and some hydro plants can supply base load. If well construction costs for natural gas are below $10 per MWh, generating electricity from natural gas is cheaper than generating power by burning coal.

Nuclear power plants can produce a huge amount of power from a single unit. However, nuclear disasters have raised concerns over the safety of nuclear power, and the capital cost of nuclear plants is very high. Hydroelectric power plants are located in areas where the potential energy from falling water can be harnessed for moving turbines and the generation of power. It may not be an economically viable single source of production where the ability to store the flow of water is limited and the load varies too much during the annual production cycle.

Generating equipment
Electric generators were known in simple forms from the discovery of electromagnetic induction in the 1830s. In general, some form of prime mover such as an engine or the turbines described above, drives a rotating magnetic field past stationary coils of wire thereby turning mechanical energy into electricity. The only commercial scale forms of electricity production that do not employ a generator are photovoltaic solar and fuel cells.

Turbines
Almost all commercial electrical power on Earth is generated with a turbine, driven by wind, water, steam or burning gas. The turbine drives a generator, thus transforming its mechanical energy into electrical energy by electromagnetic induction. There are many different methods of developing mechanical energy, including heat engines, hydro, wind and tidal power. Most electric generation is driven by heat engines.

The combustion of fossil fuels supplies most of the energy to these engines, with a significant fraction from nuclear fission and some from renewable sources. The modern steam turbine, invented by Sir Charles Parsons in 1884, currently generates about 80% of the electric power in the world using a variety of heat sources. Turbine types include: Turbines can also use other heat-transfer liquids than steam. Supercritical carbon dioxide based cycles can provide higher conversion efficiency due to faster heat exchange, higher energy density and simpler power cycle infrastructure. Supercritical carbon dioxide blends, that are currently in development, can further increase efficiency by optimizing its critical pressure and temperature points.
 * Steam
 * Water is boiled by coal burned in a thermal power plant. About 41% of all electricity is generated this way.
 * Nuclear fission heat created in a nuclear reactor creates steam. Less than 15% of electricity is generated this way.
 * Renewable energy. The steam is generated by biomass, solar thermal energy, or geothermal power.
 * Natural gas: turbines are driven directly by gases produced by combustion. Combined cycle are driven by both steam and natural gas. They generate power by burning natural gas in a gas turbine and use residual heat to generate steam. At least 20% of the world's electricity is generated by natural gas.
 * Water Energy is captured by a water turbine from the movement of water - from falling water, the rise and fall of tides or ocean thermal currents (see ocean thermal energy conversion). Currently, hydroelectric plants provide approximately 16% of the world's electricity.
 * The windmill was a very early wind turbine. In 2018 around 5% of the world's electricity was produced from wind

Although turbines are most common in commercial power generation, smaller generators can be powered by gasoline or diesel engines. These may used for backup generation or as a prime source of power within isolated villages, Furthermore, the turbine form of electricity Generation should be Imbibed by many companies that are always on high consumption of Power of Electricity without getting to pay for Power Bills.

World production

Total world generation in 2021 was 28,003 TWh, including coal (36%), gas (23%), hydro (15%), nuclear (10%), wind (6.6%), solar (3.7%), oil and other fossil fuels (3.1%), biomass (2.4%) and geothermal and other renewables (0.33%).

Production by country
China produced a third of the world's electricity in 2021, largely from coal. The United States produces half as much as China but uses far more natural gas and nuclear.

Environmental concerns
Variations between countries generating electrical power affect concerns about the environment. In France only 10% of electricity is generated from fossil fuels, the US is higher at 70% and China is at 80%. The cleanliness of electricity depends on its source. Methane leaks (from natural gas to fuel gas-fired power plants) and carbon dioxide emissions from fossil fuel-based electricity generation account for a significant portion of world greenhouse gas emissions. In the United States, fossil fuel combustion for electric power generation is responsible for 65% of all emissions of sulfur dioxide, the main component of acid rain. Electricity generation is the fourth highest combined source of NOx, carbon monoxide, and particulate matter in the US.

According to the International Energy Agency (IEA), low-carbon electricity generation needs to account for 85% of global electrical output by 2040 in order to ward off the worst effects of climate change. Like other organizations including the Energy Impact Center (EIC) and the United Nations Economic Commission for Europe (UNECE), the IEA has called for the expansion of nuclear and renewable energy to meet that objective. Some, like EIC founder Bret Kugelmass, believe that nuclear power is the primary method for decarbonizing electricity generation because it can also power direct air capture that removes existing carbon emissions from the atmosphere. Nuclear power plants can also create district heating and desalination projects, limiting carbon emissions and the need for expanded electrical output.

A fundamental issue regarding centralised generation and the current electrical generation methods in use today is the significant negative environmental effects that many of the generation processes have. Processes such as coal and gas not only release carbon dioxide as they combust, but their extraction from the ground also impacts the environment. Open pit coal mines use large areas of land to extract coal and limit the potential for productive land use after the excavation. Natural gas extraction releases large amounts of methane into the atmosphere when extracted from the ground greatly increase global greenhouse gases. Although nuclear power plants do not release carbon dioxide through electricity generation, there are risks associated with nuclear waste and safety concerns associated with the use of nuclear sources.

Per unit of electricity generated coal and gas-fired power life-cycle greenhouse gas emissions are almost always at least ten times that of other generation methods.

Centralised and distributed generation
Centralised generation is electricity generation by large-scale centralised facilities, sent through transmission lines to consumers. These facilities are usually located far away from consumers and distribute the electricity through high voltage transmission lines to a substation, where it is then distributed to consumers; the basic concept being that multi-megawatt or gigawatt scale large stations create electricity for a large number of people. The vast majority of electricity used is created from centralised generation. Most centralised power generation comes from large power plants run by fossil fuels such as coal or natural gas, though nuclear or large hydroelectricity plants are also commonly used.

Centralised generation is fundamentally the opposite of distributed generation. Distributed generation is the small-scale generation of electricity to smaller groups of consumers. This can also include independently producing electricity by either solar or wind power. In recent years distributed generation as has seen a spark in popularity due to its propensity to use renewable energy generation methods such as rooftop solar.

Technologies
Centralised energy sources are large power plants that produce huge amounts of electricity to a large number of consumers. Most power plants used in centralised generation are thermal power plants meaning that they use a fuel to heat steam to produce a pressurised gas which in turn spins a turbine and generates electricity. This is the traditional way of producing energy. This process relies on several forms of technology to produce widespread electricity, these being natural coal, gas and nuclear forms of thermal generation. More recently solar and wind have become large scale.

Natural gas
Natural gas is ignited to create pressurised gas which is used to spin turbines to generate electricity. Natural gas plants use a gas turbine where natural gas is added along with oxygen which in turn combusts and expands through the turbine to force a generator to spin.

Natural gas power plants are more efficient than coal power generation, they however contribute to climate change, but not as highly as coal generation. Not only do they produce carbon dioxide from the ignition of natural gas, the extraction of gas when mined releases a significant amount of methane into the atmosphere.

Nuclear
Nuclear power plants create electricity through steam turbines where the heat input is from the process of nuclear fission. Currently, nuclear power produces 11% of all electricity in the world. Most nuclear reactors use uranium as a source of fuel. In a process called nuclear fission, energy, in the form of heat, is released when nuclear atoms are split. Electricity is created through the use of a nuclear reactor where heat produced by nuclear fission is used to produce steam which in turn spins turbines and powers the generators. Although there are several types of nuclear reactors, all fundamentally use this process.

Normal emissions due to nuclear power plants are primarily waste heat and radioactive spent fuel. In a reactor accident, significant amounts of radioisotopes can be released to the environment, posing a long term hazard to life. This hazard has been a continuing concern of environmentalists. Accidents such as the Three Mile Island accident, Chernobyl disaster and the Fukushima nuclear disaster illustrate this problem.

Electricity generation capacity by country
The table lists 45 countries with their total electricity capacities. The data is from 2022. According to the Energy Information Administration, the total global electricity capacity in 2022 was nearly 8.9 terawatt (TW), more than four times the total global electricity capacity in 1981. The global average per-capita electricity capacity was about 1,120 watts in 2022, nearly two and a half times the global average per-capita electricity capacity in 1981.

Iceland has the highest installed capacity per capita in the world, at about 8,990 watts. All developed countries have an average per-capita electricity capacity above the global average per-capita electricity capacity, with the United Kingdom having the lowest average per-capita electricity capacity of all other developed countries.

title: Electricity generation


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Design of experiments

The design of experiments (DOE or DOX), also known as experiment design or experimental design, is the design of any task that aims to describe and explain the variation of information under conditions that are hypothesized to reflect the variation. The term is generally associated with experiments in which the design introduces conditions that directly affect the variation, but may also refer to the design of quasi-experiments, in which natural conditions that influence the variation are selected for observation.

In its simplest form, an experiment aims at predicting the outcome by introducing a change of the preconditions, which is represented by one or more independent variables, also referred to as "input variables" or "predictor variables." The change in one or more independent variables is generally hypothesized to result in a change in one or more dependent variables, also referred to as "output variables" or "response variables." The experimental design may also identify control variables that must be held constant to prevent external factors from affecting the results. Experimental design involves not only the selection of suitable independent, dependent, and control variables, but planning the delivery of the experiment under statistically optimal conditions given the constraints of available resources. There are multiple approaches for determining the set of design points (unique combinations of the settings of the independent variables) to be used in the experiment.

Main concerns in experimental design include the establishment of validity, reliability, and replicability. For example, these concerns can be partially addressed by carefully choosing the independent variable, reducing the risk of measurement error, and ensuring that the documentation of the method is sufficiently detailed. Related concerns include achieving appropriate levels of statistical power and sensitivity.

Correctly designed experiments advance knowledge in the natural and social sciences and engineering, with design of experiments methodology recognised as a key tool in the successful implementation of a Quality by Design (QbD) framework. Other applications include marketing and policy making. The study of the design of experiments is an important topic in metascience.

Statistical experiments, following Charles S. Peirce
A theory of statistical inference was developed by Charles S. Peirce in "Illustrations of the Logic of Science" (1877–1878) and "A Theory of Probable Inference" (1883), two publications that emphasized the importance of randomization-based inference in statistics.

Randomized experiments
Charles S. Peirce randomly assigned volunteers to a blinded, repeated-measures design to evaluate their ability to discriminate weights. Peirce's experiment inspired other researchers in psychology and education, which developed a research tradition of randomized experiments in laboratories and specialized textbooks in the 1800s.

Optimal designs for regression models
Charles S. Peirce also contributed the first English-language publication on an optimal design for regression models in 1876. A pioneering optimal design for polynomial regression was suggested by Gergonne in 1815. In 1918, Kirstine Smith published optimal designs for polynomials of degree six (and less).

Sequences of experiments
The use of a sequence of experiments, where the design of each may depend on the results of previous experiments, including the possible decision to stop experimenting, is within the scope of sequential analysis, a field that was pioneered by Abraham Wald in the context of sequential tests of statistical hypotheses. Herman Chernoff wrote an overview of optimal sequential designs, while adaptive designs have been surveyed by S. Zacks. One specific type of sequential design is the "two-armed bandit", generalized to the multi-armed bandit, on which early work was done by Herbert Robbins in 1952.

Fisher's principles
A methodology for designing experiments was proposed by Ronald Fisher, in his innovative books: The Arrangement of Field Experiments (1926) and The Design of Experiments (1935). Much of his pioneering work dealt with agricultural applications of statistical methods. As a mundane example, he described how to test the lady tasting tea hypothesis, that a certain lady could distinguish by flavour alone whether the milk or the tea was first placed in the cup. These methods have been broadly adapted in biological, psychological, and agricultural research.

 * Comparison
 * In some fields of study it is not possible to have independent measurements to a traceable metrology standard. Comparisons between treatments are much more valuable and are usually preferable, and often compared against a scientific control or traditional treatment that acts as baseline.

 * Randomization
 * Random assignment is the process of assigning individuals at random to groups or to different groups in an experiment, so that each individual of the population has the same chance of becoming a participant in the study. The random assignment of individuals to groups (or conditions within a group) distinguishes a rigorous, "true" experiment from an observational study or "quasi-experiment". There is an extensive body of mathematical theory that explores the consequences of making the allocation of units to treatments by means of some random mechanism (such as tables of random numbers, or the use of randomization devices such as playing cards or dice). Assigning units to treatments at random tends to mitigate confounding, which makes effects due to factors other than the treatment to appear to result from the treatment.

 * The risks associated with random allocation (such as having a serious imbalance in a key characteristic between a treatment group and a control group) are calculable and hence can be managed down to an acceptable level by using enough experimental units. However, if the population is divided into several subpopulations that somehow differ, and the research requires each subpopulation to be equal in size, stratified sampling can be used. In that way, the units in each subpopulation are randomized, but not the whole sample. The results of an experiment can be generalized reliably from the experimental units to a larger statistical population of units only if the experimental units are a random sample from the larger population; the probable error of such an extrapolation depends on the sample size, among other things.

 * Statistical replication
 * Measurements are usually subject to variation and measurement uncertainty; thus they are repeated and full experiments are replicated to help identify the sources of variation, to better estimate the true effects of treatments, to further strengthen the experiment's reliability and validity, and to add to the existing knowledge of the topic. However, certain conditions must be met before the replication of the experiment is commenced: the original research question has been published in a peer-reviewed journal or widely cited, the researcher is independent of the original experiment, the researcher must first try to replicate the original findings using the original data, and the write-up should state that the study conducted is a replication study that tried to follow the original study as strictly as possible.

 * Blocking
 * No block vs block chart.jpging is the non-random arrangement of experimental units into groups (blocks) consisting of units that are similar to one another. Blocking reduces known but irrelevant sources of variation between units and thus allows greater precision in the estimation of the source of variation under study.

 * Orthogonality
 * Orthogonality
 * Orthogonality
 * Orthogonality
 * Orthogonality concerns the forms of comparison (contrasts) that can be legitimately and efficiently carried out. Contrasts can be represented by vectors and sets of orthogonal contrasts are uncorrelated and independently distributed if the data are normal. Because of this independence, each orthogonal treatment provides different information to the others. If there are T treatments and T – 1 orthogonal contrasts, all the information that can be captured from the experiment is obtainable from the set of contrasts.

 * Multifactorial experiments
 * Use of multifactorial experiments instead of the one-factor-at-a-time method. These are efficient at evaluating the effects and possible interactions of several factors (independent variables). Analysis of experiment design is built on the foundation of the analysis of variance, a collection of models that partition the observed variance into components, according to what factors the experiment must estimate or test.

Example
This example of design experiments is attributed to Harold Hotelling, building on examples from Frank Yates. The experiments designed in this example involve combinatorial designs.

Weights of eight objects are measured using a pan balance and set of standard weights. Each weighing measures the weight difference between objects in the left pan and any objects in the right pan by adding calibrated weights to the lighter pan until the balance is in equilibrium. Each measurement has a random error. The average error is zero; the standard deviations of the probability distribution of the errors is the same number σ on different weighings; errors on different weighings are independent. Denote the true weights by

 * $$\theta_1, \dots, \theta_8.\,$$

We consider two different experiments:

 * 1) Weigh each object in one pan, with the other pan empty.  Let Xi be the measured weight of the object, for i = 1, ..., 8.
 * 2) Do the eight weighings according to the following schedule—a weighing matrix:

\begin{array}{lcc} & \text{left pan} & \text{right pan} \\ \hline \text{1st weighing:} & 1\ 2\ 3\ 4\ 5\ 6\ 7\ 8 & \text{(empty)} \\ \text{2nd:} & 1\ 2\ 3\ 8\ & 4\ 5\ 6\ 7 \\ \text{3rd:} & 1\ 4\ 5\ 8\ & 2\ 3\ 6\ 7 \\ \text{4th:} & 1\ 6\ 7\ 8\ & 2\ 3\ 4\ 5 \\ \text{5th:} & 2\ 4\ 6\ 8\ & 1\ 3\ 5\ 7 \\ \text{6th:} & 2\ 5\ 7\ 8\ & 1\ 3\ 4\ 6 \\ \text{7th:} & 3\ 4\ 7\ 8\ & 1\ 2\ 5\ 6 \\ \text{8th:} & 3\ 5\ 6\ 8\ & 1\ 2\ 4\ 7 \end{array} $$

 * Let Yi be the measured difference for i = 1, ..., 8. Then the estimated value of the weight θ1 is

 * $$\widehat{\theta}_1 = \frac{Y_1 + Y_2 + Y_3 + Y_4 - Y_5 - Y_6 - Y_7 - Y_8}{8}. $$

 * Similar estimates can be found for the weights of the other items:

\begin{align} \widehat{\theta}_2 & = \frac{Y_1 + Y_2 - Y_3 - Y_4 + Y_5 + Y_6 - Y_7 - Y_8} 8. \\[5pt] \widehat{\theta}_3 & = \frac{Y_1 + Y_2 - Y_3 - Y_4 - Y_5 - Y_6 + Y_7 + Y_8} 8. \\[5pt] \widehat{\theta}_4 & = \frac{Y_1 - Y_2 + Y_3 - Y_4 + Y_5 - Y_6 + Y_7 - Y_8} 8. \\[5pt] \widehat{\theta}_5 & = \frac{Y_1 - Y_2 + Y_3 - Y_4 - Y_5 + Y_6 - Y_7 + Y_8} 8. \\[5pt] \widehat{\theta}_6 & = \frac{Y_1 - Y_2 - Y_3 + Y_4 + Y_5 - Y_6 - Y_7 + Y_8} 8. \\[5pt] \widehat{\theta}_7 & = \frac{Y_1 - Y_2 - Y_3 + Y_4 - Y_5 + Y_6 + Y_7 - Y_8} 8. \\[5pt] \widehat{\theta}_8 & = \frac{Y_1 + Y_2 + Y_3 + Y_4 + Y_5 + Y_6 + Y_7 + Y_8} 8. \end{align} $$

The question of design of experiments is: which experiment is better?

The variance of the estimate X1 of θ1 is σ2 if we use the first experiment. But if we use the second experiment, the variance of the estimate given above is σ2/8. Thus the second experiment gives us 8 times as much precision for the estimate of a single item, and estimates all items simultaneously, with the same precision. What the second experiment achieves with eight would require 64 weighings if the items are weighed separately. However, note that the estimates for the items obtained in the second experiment have errors that correlate with each other.

Many problems of the design of experiments involve combinatorial designs, as in this example and others.

Avoiding false positives
False positive conclusions, often resulting from the pressure to publish or the author's own confirmation bias, are an inherent hazard in many fields.

Use of double-blind designs can prevent biases potentially leading to false positives in the data collection phase. When a double-blind design is used, participants are randomly assigned to experimental groups but the researcher is unaware of what participants belong to which group. Therefore, the researcher can not affect the participants' response to the intervention.

Experimental designs with undisclosed degrees of freedom are a problem, in that they can lead to conscious or unconscious "p-hacking": trying multiple things until you get the desired result. It typically involves the manipulation – perhaps unconsciously – of the process of statistical analysis and the degrees of freedom until they return a figure below the p<.05 level of statistical significance.

P-hacking can be prevented by preregistering researches, in which researchers have to send their data analysis plan to the journal they wish to publish their paper in before they even start their data collection, so no data manipulation is possible.

Another way to prevent this is taking a double-blind design to the data-analysis phase, making the study triple-blind, where the data are sent to a data-analyst unrelated to the research who scrambles up the data so there is no way to know which participants belong to before they are potentially taken away as outliers.

Clear and complete documentation of the experimental methodology is also important in order to support replication of results.

Discussion topics when setting up an experimental design
An experimental design or randomized clinical trial requires careful consideration of several factors before actually doing the experiment. An experimental design is the laying out of a detailed experimental plan in advance of doing the experiment. Some of the following topics have already been discussed in the principles of experimental design section:

 * 1) How many factors does the design have, and are the levels of these factors fixed or random?
 * 2) Are control conditions needed, and what should they be?
 * 3) Manipulation checks: did the manipulation really work?
 * 4) What are the background variables?
 * 5) What is the sample size? How many units must be collected for the experiment to be generalisable and have enough power?
 * 6) What is the relevance of interactions between factors?
 * 7) What is the influence of delayed effects of substantive factors on outcomes?
 * 8) How do response shifts affect self-report measures?
 * 9) How feasible is repeated administration of the same measurement instruments to the same units at different occasions, with a post-test and follow-up tests?
 * 10) What about using a proxy pretest?
 * 11) Are there lurking variables?
 * 12) Should the client/patient, researcher or even the analyst of the data be blind to conditions?
 * 13) What is the feasibility of subsequent application of different conditions to the same units?
 * 14) How many of each control and noise factors should be taken into account?

The independent variable of a study often has many levels or different groups. In a true experiment, researchers can have an experimental group, which is where their intervention testing the hypothesis is implemented, and a control group, which has all the same element as the experimental group, without the interventional element. Thus, when everything else except for one intervention is held constant, researchers can certify with some certainty that this one element is what caused the observed change. In some instances, having a control group is not ethical. This is sometimes solved using two different experimental groups. In some cases, independent variables cannot be manipulated, for example when testing the difference between two groups who have a different disease, or testing the difference between genders (obviously variables that would be hard or unethical to assign participants to). In these cases, a quasi-experimental design may be used.

Causal attributions
In the pure experimental design, the independent (predictor) variable is manipulated by the researcher – that is – every participant of the research is chosen randomly from the population, and each participant chosen is assigned randomly to conditions of the independent variable. Only when this is done is it possible to certify with high probability that the reason for the differences in the outcome variables are caused by the different conditions. Therefore, researchers should choose the experimental design over other design types whenever possible. However, the nature of the independent variable does not always allow for manipulation. In those cases, researchers must be aware of not certifying about causal attribution when their design doesn't allow for it. For example, in observational designs, participants are not assigned randomly to conditions, and so if there are differences found in outcome variables between conditions, it is likely that there is something other than the differences between the conditions that causes the differences in outcomes, that is – a third variable. The same goes for studies with correlational design (Adér & Mellenbergh, 2008).

Statistical control
It is best that a process be in reasonable statistical control prior to conducting designed experiments. When this is not possible, proper blocking, replication, and randomization allow for the careful conduct of designed experiments. To control for nuisance variables, researchers institute control checks as additional measures. Investigators should ensure that uncontrolled influences (e.g., source credibility perception) do not skew the findings of the study. A manipulation check is one example of a control check. Manipulation checks allow investigators to isolate the chief variables to strengthen support that these variables are operating as planned.

One of the most important requirements of experimental research designs is the necessity of eliminating the effects of spurious, intervening, and antecedent variables. In the most basic model, cause (X) leads to effect (Y). But there could be a third variable (Z) that influences (Y), and X might not be the true cause at all. Z is said to be a spurious variable and must be controlled for. The same is true for intervening variables (a variable in between the supposed cause (X) and the effect (Y)), and anteceding variables (a variable prior to the supposed cause (X) that is the true cause). When a third variable is involved and has not been controlled for, the relation is said to be a zero order relationship. In most practical applications of experimental research designs there are several causes (X1, X2, X3). In most designs, only one of these causes is manipulated at a time.

Experimental designs after Fisher
Some efficient designs for estimating several main effects were found independently and in near succession by Raj Chandra Bose and K. Kishen in 1940 at the Indian Statistical Institute, but remained little known until the Plackett–Burman designs were published in Biometrika in 1946. About the same time, C. R. Rao introduced the concepts of orthogonal arrays as experimental designs. This concept played a central role in the development of Taguchi methods by Genichi Taguchi, which took place during his visit to Indian Statistical Institute in early 1950s. His methods were successfully applied and adopted by Japanese and Indian industries and subsequently were also embraced by US industry albeit with some reservations.

In 1950, Gertrude Mary Cox and William Gemmell Cochran published the book Experimental Designs, which became the major reference work on the design of experiments for statisticians for years afterwards.

Developments of the theory of linear models have encompassed and surpassed the cases that concerned early writers. Today, the theory rests on advanced topics in linear algebra, algebra and combinatorics.

As with other branches of statistics, experimental design is pursued using both frequentist and Bayesian approaches: In evaluating statistical procedures like experimental designs, frequentist statistics studies the sampling distribution while Bayesian statistics updates a probability distribution on the parameter space.

Some important contributors to the field of experimental designs are C. S. Peirce, R. A. Fisher, F. Yates, R. C. Bose, A. C. Atkinson, R. A. Bailey, D. R. Cox, G. E. P. Box, W. G. Cochran, W. T. Federer, V. V. Fedorov, A. S. Hedayat, J. Kiefer, O. Kempthorne, J. A. Nelder, Andrej Pázman, Friedrich Pukelsheim, D. Raghavarao, C. R. Rao, Shrikhande S. S., J. N. Srivastava, William J. Studden, G. Taguchi and H. P. Wynn.

The textbooks of D. Montgomery, R. Myers, and G. Box/W. Hunter/J.S. Hunter have reached generations of students and practitioners. Furthermore, there is ongoing discussion of experimental design in the context of model building for models either static or dynamic models, also known as system identification.

Human participant constraints
Laws and ethical considerations preclude some carefully designed experiments with human subjects. Legal constraints are dependent on jurisdiction. Constraints may involve institutional review boards, informed consent and confidentiality affecting both clinical (medical) trials and behavioral and social science experiments. In the field of toxicology, for example, experimentation is performed on laboratory animals with the goal of defining safe exposure limits for humans. Balancing the constraints are views from the medical field. Regarding the randomization of patients, "... if no one knows which therapy is better, there is no ethical imperative to use one therapy or another." (p 380) Regarding experimental design, "...it is clearly not ethical to place subjects at risk to collect data in a poorly designed study when this situation can be easily avoided...". (p 393)

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

title: Design of experiments


text:
Empirical research

Empirical research is research using empirical evidence. It is also a way of gaining knowledge by means of direct and indirect observation or experience. Empiricism values some research more than other kinds. Empirical evidence (the record of one's direct observations or experiences) can be analyzed quantitatively or qualitatively. Quantifying the evidence or making sense of it in qualitative form, a researcher can answer empirical questions, which should be clearly defined and answerable with the evidence collected (usually called data). Research design varies by field and by the question being investigated. Many researchers combine qualitative and quantitative forms of analysis to better answer questions that cannot be studied in laboratory settings, particularly in the social sciences and in education.

In some fields, quantitative research may begin with a research question (e.g., "Does listening to vocal music during the learning of a word list have an effect on later memory for these words?") which is tested through experimentation. Usually, the researcher has a certain theory regarding the topic under investigation. Based on this theory, statements or hypotheses will be proposed (e.g., "Listening to vocal music has a negative effect on learning a word list."). From these hypotheses, predictions about specific events are derived (e.g., "People who study a word list while listening to vocal music will remember fewer words on a later memory test than people who study a word list in silence."). These predictions can then be tested with a suitable experiment. Depending on the outcomes of the experiment, the theory on which the hypotheses and predictions were based will be supported or not, or may need to be modified and then subjected to further testing.

Terminology
The term empirical was originally used to refer to certain ancient Greek practitioners of medicine who rejected adherence to the dogmatic doctrines of the day, preferring instead to rely on the observation of phenomena as perceived in experience. Later empiricism referred to a theory of knowledge in philosophy which adheres to the principle that knowledge arises from experience and evidence gathered specifically using the senses. In scientific use, the term empirical refers to the gathering of data using only evidence that is observable by the senses or in some cases using calibrated scientific instruments. What early philosophers described as empiricist and empirical research have in common is the dependence on observable data to formulate and test theories and come to conclusions.

Usage
The researcher attempts to describe accurately the interaction between the instrument (or the human senses) and the entity being observed. If instrumentation is involved, the researcher is expected to calibrate his/her instrument by applying it to known standard objects and documenting the results before applying it to unknown objects. In other words, it describes the research that has not taken place before and their results.

In practice, the accumulation of evidence for or against any particular theory involves planned research designs for the collection of empirical data, and academic rigor plays a large part of judging the merits of research design. Several typologies for such designs have been suggested, one of the most popular of which comes from Campbell and Stanley. They are responsible for popularizing the widely cited distinction among pre-experimental, experimental, and quasi-experimental designs and are staunch advocates of the central role of randomized experiments in educational research.

Scientific research
Accurate analysis of data using standardized statistical methods in scientific studies is critical to determining the validity of empirical research. Statistical formulas such as regression, uncertainty coefficient, t-test, chi square, and various types of ANOVA (analyses of variance) are fundamental to forming logical, valid conclusions. If empirical data reach significance under the appropriate statistical formula, the research hypothesis is supported. If not, the null hypothesis is supported (or, more accurately, not rejected), meaning no effect of the independent variable(s) was observed on the dependent variable(s).

The result of empirical research using statistical hypothesis testing is never proof. It can only support a hypothesis, reject it, or do neither. These methods yield only probabilities. Among scientific researchers, empirical evidence (as distinct from empirical research) refers to objective evidence that appears the same regardless of the observer. For example, a thermometer will not display different temperatures for each individual who observes it. Temperature, as measured by an accurate, well calibrated thermometer, is empirical evidence. By contrast, non-empirical evidence is subjective, depending on the observer. Following the previous example, observer A might truthfully report that a room is warm, while observer B might truthfully report that the same room is cool, though both observe the same reading on the thermometer. The use of empirical evidence negates this effect of personal (i.e., subjective) experience or time.

The varying perception of empiricism and rationalism shows concern with the limit to which there is dependency on experience of sense as an effort of gaining knowledge. According to rationalism, there are a number of different ways in which sense experience is gained independently for the knowledge and concepts. According to empiricism, sense experience is considered as the main source of every piece of knowledge and the concepts. In general, rationalists are known for the development of their own views following two different way. First, the key argument can be placed that there are cases in which the content of knowledge or concepts end up outstripping the information. This outstripped information is provided by the sense experience (Hjørland, 2010, 2). Second, there is construction of accounts as to how reasoning helps in the provision of addition knowledge about a specific or broader scope. Empiricists are known to be presenting complementary senses related to thought.

First, there is development of accounts of how there is provision of information by experience that is cited by rationalists. This is insofar for having it in the initial place. At times, empiricists tend to be opting skepticism as an option of rationalism. If experience is not helpful in the provision of knowledge or concept cited by rationalists, then they do not exist (Pearce, 2010, 35). Second, empiricists hold the tendency of attacking the accounts of rationalists while considering reasoning to be an important source of knowledge or concepts. The overall disagreement between empiricists and rationalists shows primary concerns in how there is gaining of knowledge with respect to the sources of knowledge and concept. In some of the cases, disagreement at the point of gaining knowledge results in the provision of conflicting responses to other aspects as well. There might be a disagreement in the overall feature of warrant, while limiting the knowledge and thought. Empiricists are known for sharing the view that there is no existence of innate knowledge and rather that is derivation of knowledge out of experience. These experiences are either reasoned using the mind or sensed through the five senses human possess (Bernard, 2011, 5). On the other hand, rationalists are known to be sharing the view that there is existence of innate knowledge and this is different for the objects of innate knowledge being chosen.

In order to follow rationalism, there must be adoption of one of the three claims related to the theory that are deduction or intuition, innate knowledge, and innate concept. The more there is removal of concept from mental operations and experience, there can be performance over experience with increased plausibility in being innate. Further ahead, empiricism in context with a specific subject provides a rejection of the corresponding version related to innate knowledge and deduction or intuition (Weiskopf, 2008, 16). Insofar as there is acknowledgement of concepts and knowledge within the area of subject, the knowledge has major dependence on experience through human senses.

Empirical cycle
A.D. de Groot's empirical cycle:
 * 1) Observation: The observation of a phenomenon and inquiry concerning its causes.
 * 2) Induction: The formulation of hypotheses - generalized explanations for the phenomenon.
 * 3) Deduction: The formulation of experiments that will test the hypotheses (i.e. confirm them if true, refute them if false).
 * 4) Testing: The procedures by which the hypotheses are tested and data are collected.
 * 5) Evaluation: The interpretation of the data and the formulation of a theory - an abductive argument that presents the results of the experiment as the most reasonable explanation for the phenomenon.

title: Empirical research


text:
Engineering statistics

Engineering statistics combines engineering and statistics using scientific methods for analyzing data. Engineering statistics involves data concerning manufacturing processes such as: component dimensions, tolerances, type of material, and fabrication process control. There are many methods used in engineering analysis and they are often displayed as histograms to give a visual of the data as opposed to being just numerical. Examples of methods are:
 * 1) Design of Experiments (DOE) is a methodology for formulating scientific and engineering problems using statistical models. The protocol specifies a randomization procedure for the experiment and specifies the primary data-analysis, particularly in hypothesis testing. In a secondary analysis, the statistical analyst further examines the data to suggest other questions and to help plan future experiments. In engineering applications, the goal is often to optimize a process or product, rather than to subject a scientific hypothesis to test of its predictive adequacy.  The use of optimal (or near optimal) designs reduces the cost of experimentation.
 * 2) Quality control and process control use statistics as a tool to manage conformance to specifications of manufacturing processes and their products.
 * 3) Time and methods engineering use statistics to study repetitive operations in manufacturing in order to set standards and find optimum (in some sense) manufacturing procedures.
 * 4) Reliability engineering which measures the ability of a system to perform for its intended function (and time) and has tools for improving performance.
 * 5) Probabilistic design involving the use of probability in product and system design
 * 6) System identification uses statistical methods to build mathematical models of dynamical systems from measured data. System identification also includes the optimal design of experiments for efficiently generating informative data for fitting such models.

History
Engineering statistics dates back to 1000 B.C. when the Abacus was developed as means to calculate numerical data. In the 1600s, the development of information processing to systematically analyze and process data began. In 1654, the Slide Rule technique was developed by Robert Bissaker for advanced data calculations. In 1833, a British mathematician named Charles Babbage designed the idea of an automatic computer which inspired developers at Harvard University and IBM to design the first mechanical automatic-sequence-controlled calculator called MARK I. The integration of computers and calculators into the industry brought about a more efficient means of analyzing data and the beginning of engineering statistics.

Factorial Experimental Design
A factorial experiment is one where, contrary to the standard experimental philosophy of changing only one independent variable and holding everything else constant, multiple independent variables are tested at the same time. With this design, statistical engineers can see both the direct effects of one independent variable (main effect), as well as potential interaction effects that arise when multiple independent variables provide a different result when together than either would on its own.

Six Sigma
Six Sigma is a set of techniques to improve the reliability of a manufacturing process. Ideally, all products will have the exact same specifications equivalent to what was desired, but countless imperfections of real-world manufacturing makes this impossible. The as-built specifications of a product are assumed to be centered around a mean, with each individual product deviating some amount away from that mean in a normal distribution. The goal of Six Sigma is to ensure that the acceptable specification limits are six standard deviations away from the mean of the distribution; in other words, that each step of the manufacturing process has at most a 0.00034% chance of producing a defect.

title: Engineering statistics


text:
Edgar Allan Poe

Edgar Allan Poe (January 19, 1809 – October 7, 1849) was an American writer, poet, author, editor, and literary critic who is best known for his poetry and short stories, particularly his tales of mystery and the macabre. He is widely regarded as a central figure of Romanticism and Gothic fiction in the United States, and of American literature. Poe was one of the country's earliest practitioners of the short story, and is considered the inventor of the detective fiction genre, as well as a significant contributor to the emerging genre of science fiction. He is the first well-known American writer to earn a living by writing alone, which resulted in a financially difficult life and career.

Poe was born in Boston and was the second child of actors David and Elizabeth "Eliza" Poe. His father abandoned the family in 1810, and when his mother died the following year, Poe was taken in by John and Frances Allan of Richmond, Virginia. They never formally adopted him, but he was with them well into young adulthood. He attended the University of Virginia but left after a year due to a lack of money. He quarreled with John Allan over the funds for his education and his gambling debts. In 1827, having enlisted in the United States Army under an assumed name, he published his first collection, Tamerlane and Other Poems, credited only to "a Bostonian". Poe and Allan reached a temporary rapprochement after the death of Allan's wife in 1829. Poe later failed as an officer cadet at West Point, declared a firm wish to be a poet and writer, and parted ways with Allan.

Poe switched his focus to prose, and spent the next several years working for literary journals and periodicals, becoming known for his own style of literary criticism. His work forced him to move between several cities, including Baltimore, Philadelphia, and New York City. In 1836, when he was 27, he married his 13-year-old cousin, Virginia Clemm. She died of tuberculosis in 1847. In January 1845, he published his poem "The Raven" to instant success. He planned for years to produce his own journal The Penn, later renamed The Stylus. But before it began publishing, Poe died in Baltimore in 1849, aged 40, under mysterious circumstances. The cause of his death remains unknown and has been variously attributed to many causes, including disease, alcoholism, substance abuse, and suicide.

Poe and his works influenced literature worldwide and specialized fields such as cosmology and cryptography. He and his work appear throughout popular culture in literature, music, films, and television. Several of his homes are dedicated museums. The Mystery Writers of America presents an annual Edgar Award for distinguished work in the mystery genre.

Early life and education
Edgar Poe was born in Boston, Massachusetts, on January 19, 1809, the second child of American actor David Poe Jr. and English-born actress Elizabeth Arnold Hopkins Poe. He had an elder brother, Henry, and a younger sister, Rosalie. Their grandfather, David Poe, had emigrated from County Cavan, Ireland, around 1750.

His father abandoned the family in 1810, and his mother died a year later from pulmonary tuberculosis. Poe was then taken into the home of John Allan, a successful merchant in Richmond, Virginia, who dealt in a variety of goods, including cloth, wheat, tombstones, tobacco, and slaves. The Allans served as a foster family and gave him the name "Edgar Allan Poe", although they never formally adopted him.

The Allan family had Poe baptized into the Episcopal Church in 1812. John Allan alternately spoiled and aggressively disciplined his foster son. The family sailed to the United Kingdom in 1815, and Poe attended the grammar school for a short period in Irvine, Ayrshire, Scotland, where Allan was born, before rejoining the family in London in 1816. There he studied at a boarding school in Chelsea until summer 1817. He was subsequently entered at the Reverend John Bransby's Manor House School at Stoke Newington, then a suburb 4 mi north of London.

Poe moved with the Allans back to Richmond in 1820. In 1824, he served as the lieutenant of the Richmond youth honor guard as the city celebrated the visit of the Marquis de Lafayette. In March 1825, Allan's uncle and business benefactor William Galt died, who was said to be one of the wealthiest men in Richmond, leaving Allan several acres of real estate. The inheritance was estimated at $750,000. By summer 1825, Allan celebrated his expansive wealth by purchasing a two-story brick house called Moldavia.

Poe may have become engaged to Sarah Elmira Royster before he registered at the University of Virginia in February 1826 to study ancient and modern languages. The university was in its infancy, established on the ideals of its founder, Thomas Jefferson. It had strict rules against gambling, horses, guns, tobacco, and alcohol, but these rules were mostly ignored. Jefferson enacted a system of student self-government, allowing students to choose their own studies, make their own arrangements for boarding, and report all wrongdoing to the faculty. The unique system was still in chaos, and there was a high dropout rate. During his time there, Poe lost touch with Royster and also became estranged from his foster father over gambling debts. He claimed that Allan had not given him sufficient money to register for classes, purchase texts, and procure and furnish a dormitory. Allan did send additional money and clothes, but Poe's debts increased. Poe gave up on the university after a year but did not feel welcome returning to Richmond, especially when he learned that his sweetheart Royster had married another man, Alexander Shelton. He traveled to Boston in April 1827, sustaining himself with odd jobs as a clerk and newspaper writer, and started using the pseudonym Henri Le Rennet during this period.

Military career
Poe was unable to support himself, so he enlisted in the United States Army as a private on May 27, 1827, using the name "Edgar A. Perry". He claimed that he was 22 years old even though he was 18. He first served at Fort Independence in Boston Harbor for five dollars a month. That year, he released his first book, a 40-page collection of poetry titled Tamerlane and Other Poems, attributed with the byline "by a Bostonian". Only 50 copies were printed, and the book received virtually no attention. Poe's regiment was posted to Fort Moultrie in Charleston, South Carolina, and traveled by ship on the brig Waltham on November 8, 1827. Poe was promoted to "artificer", an enlisted tradesman who prepared shells for artillery, and had his monthly pay doubled. He served for two years and attained the rank of Sergeant Major for Artillery, the highest rank that a non-commissioned officer could achieve; he then sought to end his five-year enlistment early. He revealed his real name and his circumstances to his commanding officer, Lieutenant Howard, who would allow Poe to be discharged only if he reconciled with Allan. Poe wrote a letter to Allan, who was unsympathetic and spent several months ignoring Poe's pleas; Allan may not have written to Poe even to make him aware of his foster mother's illness. Frances Allan died on February 28, 1829, and Poe visited the day after her burial. Perhaps softened by his wife's death, Allan agreed to support Poe's attempt to be discharged in order to receive an appointment to the United States Military Academy at West Point, New York.

Poe was finally discharged on April 15, 1829, after securing a replacement to finish his enlisted term for him. Before entering West Point, he moved to Baltimore for a time to stay with his widowed aunt Maria Clemm, her daughter Virginia Eliza Clemm (Poe's first cousin), his brother Henry, and his invalid grandmother Elizabeth Cairnes Poe. In September of that year, Poe received "the very first words of encouragement I ever remember to have heard" in a review of his poetry by influential critic John Neal, prompting Poe to dedicate one of the poems to Neal in his second book Al Aaraaf, Tamerlane and Minor Poems, published in Baltimore in 1829.

Poe traveled to West Point and matriculated as a cadet on July 1, 1830. In October 1830, Allan married his second wife Louisa Patterson. The marriage and bitter quarrels with Poe over the children born to Allan out of extramarital affairs led to the foster father finally disowning Poe. Poe decided to leave West Point by purposely getting court-martialed. On February 8, 1831, he was tried for gross neglect of duty and disobedience of orders for refusing to attend formations, classes, or church. He tactically pleaded not guilty to induce dismissal, knowing that he would be found guilty.

Poe left for New York in February 1831 and released a third volume of poems, simply titled Poems. The book was financed with help from his fellow cadets at West Point, many of whom donated 75 cents to the cause, raising a total of $170. They may have been expecting verses similar to the satirical ones Poe had written about commanding officers. It was printed by Elam Bliss of New York, labeled as "Second Edition", and including a page saying, "To the U.S. Corps of Cadets this volume is respectfully dedicated". The book once again reprinted the long poems "Tamerlane" and "Al Aaraaf" but also six previously unpublished poems, including early versions of "To Helen", "Israfel", and "The City in the Sea". Poe returned to Baltimore to his aunt, brother, and cousin in March 1831. His elder brother Henry had been in ill health, in part due to problems with alcoholism, and he died on August 1, 1831.

Publishing career
After his brother's death, Poe began more earnest attempts to start his career as a writer, but he chose a difficult time in American publishing to do so. He was one of the first Americans to live by writing alone and was hampered by the lack of an international copyright law. American publishers often produced unauthorized copies of British works rather than paying for new work by Americans. The industry was also particularly hurt by the Panic of 1837. There was a booming growth in American periodicals around this time, fueled in part by new technology, but many did not last beyond a few issues. Publishers often refused to pay their writers or paid them much later than they promised, and Poe repeatedly resorted to humiliating pleas for money and other assistance.

After his early attempts at poetry, Poe had turned his attention to prose, likely based on John Neal's critiques in The Yankee magazine. He placed a few stories with a Philadelphia publication and began work on his only drama Politian. The Baltimore Saturday Visiter awarded him a prize in October 1833 for his short story "MS. Found in a Bottle". The story brought him to the attention of John P. Kennedy, a Baltimorean of considerable means who helped Poe place some of his stories and introduced him to Thomas W. White, editor of the Southern Literary Messenger in Richmond.

In 1835, Poe became assistant editor of the Southern Literary Messenger, but White discharged him within a few weeks for being drunk on the job. Poe returned to Baltimore, where he obtained a license to marry his cousin Virginia on September 22, 1835, though it is unknown if they were married at that time. He was 26 and she was 13.

Poe was reinstated by White after promising good behavior, and he returned to Richmond with Virginia and her mother. He remained at the Messenger until January 1837. During this period, Poe claimed that its circulation increased from 700 to 3,500. He published several poems, book reviews, critiques, and stories in the paper. On May 16, 1836, he and Virginia held a Presbyterian wedding ceremony performed by Amasa Converse at their Richmond boarding house, with a witness falsely attesting Clemm's age as 21.

Philadelphia
In 1838, Poe relocated to Philadelphia, where he lived at four different residences between 1838 and 1844, one of which at 532 N. 7th Street has been preserved as a National Historic Landmark.

That same year, Poe's novel The Narrative of Arthur Gordon Pym of Nantucket was published and widely reviewed. In the summer of 1839, he became assistant editor of Burton's Gentleman's Magazine. He published numerous articles, stories, and reviews, enhancing his reputation as a trenchant critic which he had established at the Messenger. Also in 1839, the collection Tales of the Grotesque and Arabesque was published in two volumes, though he made little money from it and it received mixed reviews.

In June 1840, Poe published a prospectus announcing his intentions to start his own journal called The Stylus, although he originally intended to call it The Penn, since it would have been based in Philadelphia. He bought advertising space for his prospectus in the June 6, 1840, issue of Philadelphia's Saturday Evening Post: "Prospectus of the Penn Magazine, a Monthly Literary journal to be edited and published in the city of Philadelphia by Edgar A. Poe." The journal was never produced before Poe's death.

Poe left Burton's after about a year and found a position as writer and co-editor at Graham's Magazine, a successful monthly publication. In the last number of Graham's for 1841, Poe was among the co-signatories to an editorial note of celebration of the tremendous success the magazine had achieved in the past year: "Perhaps the editors of no magazine, either in America or in Europe, ever sat down, at the close of a year, to contemplate the progress of their work with more satisfaction than we do now. Our success has been unexampled, almost incredible. We may assert without fear of contradiction that no periodical ever witnessed the same increase during so short a period."

Around this time, Poe attempted to secure a position in the administration of John Tyler, claiming that he was a member of the Whig Party. He hoped to be appointed to the United States Custom House in Philadelphia with help from President Tyler's son Robert, an acquaintance of Poe's friend Frederick Thomas. Poe failed to show up for a meeting with Thomas to discuss the appointment in mid-September 1842, claiming to have been sick, though Thomas believed that he had been drunk. Poe was promised an appointment, but all positions were filled by others.

One evening in January 1842, Virginia showed the first signs of consumption, or tuberculosis, while singing and playing the piano, which Poe described as breaking a blood vessel in her throat. She only partially recovered, and Poe began to drink more heavily under the stress of her illness. He left Graham's and attempted to find a new position, for a time angling for a government post. He returned to New York where he worked briefly at the Evening Mirror before becoming editor of the Broadway Journal, and later its owner. There Poe alienated himself from other writers by publicly accusing Henry Wadsworth Longfellow of plagiarism, though Longfellow never responded. On January 29, 1845, Poe's poem "The Raven" appeared in the Evening Mirror and became a popular sensation. It made Poe a household name almost instantly, though he was paid only $9 for its publication. It was concurrently published in The American Review: A Whig Journal under the pseudonym "Quarles".

The Bronx
The Broadway Journal failed in 1846, and Poe moved to a cottage in Fordham, New York, in the Bronx. That home, now known as the Edgar Allan Poe Cottage, was relocated in later years to a park near the southeast corner of the Grand Concourse and Kingsbridge Road. Nearby, Poe befriended the Jesuits at St. John's College, now Fordham University. Virginia died at the cottage on January 30, 1847. Biographers and critics often suggest that Poe's frequent theme of the "death of a beautiful woman" stems from the repeated loss of women throughout his life, including his wife.

Poe was increasingly unstable after his wife's death. He attempted to court poet Sarah Helen Whitman, who lived in Providence, Rhode Island. Their engagement failed, purportedly because of Poe's drinking and erratic behavior. There is also strong evidence that Whitman's mother intervened and did much to derail the relationship. Poe then returned to Richmond and resumed a relationship with his childhood sweetheart Sarah Elmira Royster.

Death
On October 3, 1849, Poe was found semiconscious in Baltimore, "in great distress, and... in need of immediate assistance", according to Joseph W. Walker, who found him. He was taken to the Washington Medical College, where he died on Sunday, October 7, 1849, at 5:00 in the morning. Poe was not coherent long enough to explain how he came to be in his dire condition and why he was wearing clothes that were not his own. He is said to have repeatedly called out the name "Reynolds" on the night before his death, though it is unclear to whom he was referring. His attending physician said that Poe's final words were, "Lord help my poor soul". All of the relevant medical records have been lost, including Poe's death certificate.

Newspapers at the time reported Poe's death as "congestion of the brain" or "cerebral inflammation", common euphemisms for death from disreputable causes such as alcoholism. The actual cause of death remains a mystery. Speculation has included delirium tremens, heart disease, epilepsy, syphilis, meningeal inflammation, cholera, carbon monoxide poisoning, and rabies. One theory dating from 1872 suggests that Poe's death resulted from cooping, a form of electoral fraud in which citizens were forced to vote for a particular candidate, sometimes leading to violence and even murder.

Griswold's memoir
Immediately after Poe's death, his literary rival Rufus Wilmot Griswold wrote a slanted high-profile obituary under a pseudonym, filled with falsehoods that cast Poe as a lunatic, and which described him as a person who "walked the streets, in madness or melancholy, with lips moving in indistinct curses, or with eyes upturned in passionate prayers, (never for himself, for he felt, or professed to feel, that he was already damned)".

The long obituary appeared in the New York Tribune, signed "Ludwig" on the day that Poe was buried in Baltimore. It was further published throughout the country. The obituary began, "Edgar Allan Poe is dead. He died in Baltimore the day before yesterday. This announcement will startle many, but few will be grieved by it." "Ludwig" was soon identified as Griswold, an editor, critic, and anthologist who had borne a grudge against Poe since 1842. Griswold somehow became Poe's literary executor and attempted to destroy his enemy's reputation after his death.

Griswold wrote a biographical article of Poe called "Memoir of the Author", which he included in an 1850 volume of the collected works. There he depicted Poe as a depraved, drunken, drug-addled madman and included Poe's letters as evidence. Many of his claims were either lies or distortions; for example, it is seriously disputed that Poe was a drug addict. Griswold's book was denounced by those who knew Poe well, including John Neal, who published an article defending Poe and attacking Griswold as a "Rhadamanthus, who is not to be bilked of his fee, a thimble-full of newspaper notoriety". Griswold's book nevertheless became a popularly accepted biographical source. This was in part because it was the only full biography available and was widely reprinted, and in part because readers thrilled at the thought of reading works by an "evil" man. Letters that Griswold presented as proof were later revealed as forgeries.

Genres
Poe's best-known fiction works are Gothic horror, adhering to the genre's conventions to appeal to the public taste. His most recurring themes deal with questions of death, including its physical signs, the effects of decomposition, concerns of premature burial, the reanimation of the dead, and mourning. Many of his works are generally considered part of the dark romanticism genre, a literary reaction to transcendentalism which Poe strongly disliked. He referred to followers of the transcendental movement as "Frog-Pondians", after the pond on Boston Common, and ridiculed their writings as "metaphor—run mad," lapsing into "obscurity for obscurity's sake" or "mysticism for mysticism's sake". Poe once wrote in a letter to Thomas Holley Chivers that he did not dislike transcendentalists, "only the pretenders and sophists among them".

Beyond horror, Poe also wrote satires, humor tales, and hoaxes. For comic effect, he used irony and ludicrous extravagance, often in an attempt to liberate the reader from cultural conformity. "Metzengerstein" is the first story that Poe is known to have published and his first foray into horror, but it was originally intended as a burlesque satirizing the popular genre. Poe also reinvented science fiction, responding in his writing to emerging technologies such as hot air balloons in "The Balloon-Hoax".

Poe wrote much of his work using themes aimed specifically at mass-market tastes. To that end, his fiction often included elements of popular pseudosciences, such as phrenology and physiognomy.

Literary theory
Poe's writing reflects his literary theories, which he presented in his criticism and also in essays such as "The Poetic Principle". He disliked didacticism and allegory, though he believed that meaning in literature should be an undercurrent just beneath the surface. Works with obvious meanings, he wrote, cease to be art. He believed that work of quality should be brief and focus on a specific single effect. To that end, he believed that the writer should carefully calculate every sentiment and idea.

Poe describes his method in writing "The Raven" in the essay "The Philosophy of Composition", and he claims to have strictly followed this method. It has been questioned whether he really followed this system, however. T. S. Eliot said: "It is difficult for us to read that essay without reflecting that if Poe plotted out his poem with such calculation, he might have taken a little more pains over it: the result hardly does credit to the method." Biographer Joseph Wood Krutch described the essay as "a rather highly ingenious exercise in the art of rationalization".

Influence
During his lifetime, Poe was mostly recognized as a literary critic. Fellow critic James Russell Lowell called him "the most discriminating, philosophical, and fearless critic upon imaginative works who has written in America", suggesting—rhetorically—that he occasionally used prussic acid instead of ink. Poe's caustic reviews earned him the reputation of being a "tomahawk man". A favorite target of Poe's criticism was Boston's acclaimed poet Henry Wadsworth Longfellow, who was often defended by his literary friends in what was later called "The Longfellow War". Poe accused Longfellow of "the heresy of the didactic", writing poetry that was preachy, derivative, and thematically plagiarized. Poe correctly predicted that Longfellow's reputation and style of poetry would decline, concluding, "We grant him high qualities, but deny him the Future".

Poe was also known as a writer of fiction and became one of the first American authors of the 19th century to become more popular in Europe than in the United States. Poe is particularly respected in France, in part due to early translations by Charles Baudelaire. Baudelaire's translations became definitive renditions of Poe's work in Continental Europe.

Poe's early detective fiction tales featuring C. Auguste Dupin laid the groundwork for future detectives in literature. Sir Arthur Conan Doyle said, "Each [of Poe's detective stories] is a root from which a whole literature has developed.... Where was the detective story until Poe breathed the breath of life into it?" The Mystery Writers of America have named their awards for excellence in the genre the "Edgars". Poe's work also influenced science fiction, notably Jules Verne, who wrote a sequel to Poe's novel The Narrative of Arthur Gordon Pym of Nantucket called An Antarctic Mystery, also known as The Sphinx of the Ice Fields. Science fiction author H. G. Wells noted, "Pym tells what a very intelligent mind could imagine about the south polar region a century ago". In 2013, The Guardian cited Pym as one of the greatest novels ever written in the English language, and noted its influence on later authors such as Doyle, Henry James, B. Traven, and David Morrell.

Horror author and historian H. P. Lovecraft was heavily influenced by Poe's horror tales, dedicating an entire section of his long essay, "Supernatural Horror in Literature", to his influence on the genre. In his letters, Lovecraft described Poe as his "God of Fiction". Lovecraft's earlier stories express a significant influence from Poe. A later work, At the Mountains of Madness, quotes him and was influenced by The Narrative of Arthur Gordon Pym of Nantucket. Lovecraft also made extensive use of Poe's unity of effect in his fiction. Alfred Hitchcock once said, "It's because I liked Edgar Allan Poe's stories so much that I began to make suspense films". Many references to Poe's works are present in Vladimir Nabokov's novels.

Like many famous artists, Poe's works have spawned imitators. One trend among imitators of Poe has been claims by clairvoyants or psychics to be "channeling" poems from Poe's spirit. One of the most notable of these was Lizzie Doten, who published Poems from the Inner Life in 1863, in which she claimed to have "received" new compositions by Poe's spirit. The compositions were re-workings of famous Poe poems such as "The Bells", but which reflected a new, positive outlook.

Poe has also received criticism. This is partly because of the negative perception of his personal character and its influence upon his reputation. William Butler Yeats was occasionally critical of Poe and once called him "vulgar". Transcendentalist Ralph Waldo Emerson reacted to "The Raven" by saying, "I see nothing in it", and derisively referred to Poe as "the jingle man". Aldous Huxley wrote that Poe's writing "falls into vulgarity" by being "too poetical"—the equivalent of wearing a diamond ring on every finger.

It is believed that only twelve copies have survived of Poe's first book Tamerlane and Other Poems. In December 2009, one copy sold at Christie's auctioneers in New York City for $662,500, a record price paid for a work of American literature.

Physics and cosmology
Eureka: A Prose Poem, an essay written in 1848, included a cosmological theory that presaged the Big Bang theory by 80 years, as well as the first plausible solution to Olbers' paradox. Poe eschewed the scientific method in Eureka and instead wrote from pure intuition. For this reason, he considered it a work of art, not science, but insisted that it was still true and considered it to be his career masterpiece. Even so, Eureka is full of scientific errors. In particular, Poe's suggestions ignored Newtonian principles regarding the density and rotation of planets.

Cryptography
Poe had a keen interest in cryptography. He had placed a notice of his abilities in the Philadelphia paper Alexander's Weekly (Express) Messenger, inviting submissions of ciphers which he proceeded to solve. In July 1841, Poe had published an essay called "A Few Words on Secret Writing" in Graham's Magazine. Capitalizing on public interest in the topic, he wrote "The Gold-Bug" incorporating ciphers as an essential part of the story. Poe's success with cryptography relied not so much on his deep knowledge of that field (his method was limited to the simple substitution cryptogram) as on his knowledge of the magazine and newspaper culture. His keen analytical abilities, which were so evident in his detective stories, allowed him to see that the general public was largely ignorant of the methods by which a simple substitution cryptogram can be solved, and he used this to his advantage. The sensation that Poe created with his cryptography stunts played a major role in popularizing cryptograms in newspapers and magazines.

Two ciphers he published in 1841 under the name "W. B. Tyler" were not solved until 1992 and 2000 respectively. One was a quote from Joseph Addison's play Cato; the other is probably based on a poem by Hester Thrale.

Poe had an influence on cryptography beyond increasing public interest during his lifetime. William Friedman, America's foremost cryptologist, was heavily influenced by Poe. Friedman's initial interest in cryptography came from reading "The Gold-Bug" as a child, an interest that he later put to use in deciphering Japan's PURPLE code during World War II.

As a character
The historical Edgar Allan Poe has appeared as a fictionalized character, often in order to represent the "mad genius" or "tormented artist" and in order to exploit his personal struggles. Many such depictions also blend in with characters from his stories, suggesting that Poe and his characters share identities. Often, fictional depictions of Poe use his mystery-solving skills in such novels as The Poe Shadow by Matthew Pearl.

Preserved homes, landmarks, and museums
No childhood home of Poe is still standing, including the Allan family's Moldavia estate. The oldest standing home in Richmond, the Old Stone House, is in use as the Edgar Allan Poe Museum, though Poe never lived there. The collection includes many items that Poe used during his time with the Allan family, and also features several rare first printings of Poe works. 13 West Range is the dorm room that Poe is believed to have used while studying at the University of Virginia in 1826; it is preserved and available for visits. Its upkeep is overseen by a group of students and staff known as the Raven Society.

The earliest surviving home in which Poe lived is at 203 North Amity St. in Baltimore, which is preserved as the Edgar Allan Poe House and Museum. Poe is believed to have lived in the home at the age of 23 when he first lived with Maria Clemm and Virginia and possibly his grandmother and possibly his brother William Henry Leonard Poe. It is open to the public and is also the home of the Edgar Allan Poe Society.

While in Philadelphia between 1838 and 1844, Poe lived at at least four different residences, including the Indian Queen Hotel at 15 S. 4th Street, at a residence at 16th and Locust Streets, at 2502 Fairmount Street, and then in the Spring Garden section of the city at 532 N. 7th Street, a residence that has been preserved by the National Park Service as the Edgar Allan Poe National Historic Site. Poe's final home in Bronx, New York City, is preserved as the Edgar Allan Poe Cottage.

In Boston, a commemorative plaque on Boylston Street is several blocks away from the actual location of Poe's birth. The house which was his birthplace at 62 Carver Street no longer exists; also, the street has since been renamed "Charles Street South". A "square" at the intersection of Broadway, Fayette, and Carver Streets had once been named in his honor, but it disappeared when the streets were rearranged. In 2009, the intersection of Charles and Boylston Streets (two blocks north of his birthplace) was designated "Edgar Allan Poe Square".

In March 2014, fundraising was completed for construction of a permanent memorial sculpture, known as Poe Returning to Boston, at this location. The winning design by Stefanie Rocknak depicts a life-sized Poe striding against the wind, accompanied by a flying raven; his suitcase lid has fallen open, leaving a "paper trail" of literary works embedded in the sidewalk behind him. The public unveiling on October 5, 2014, was attended by former U.S. poet laureate Robert Pinsky.

Other Poe landmarks include a building on the Upper West Side, where Poe temporarily lived when he first moved to New York City. A plaque suggests that Poe wrote "The Raven" here. On Sullivan's Island in Charleston County, South Carolina, the setting of Poe's tale "The Gold-Bug" and where Poe served in the Army in 1827 at Fort Moultrie, there is a restaurant called Poe's Tavern. In the Fell's Point section of Baltimore, a bar still stands where legend says that Poe was last seen drinking before his death. Known as "The Horse You Came in On", local lore insists that a ghost whom they call "Edgar" haunts the rooms above.

Photographs
Early daguerreotypes of Poe continue to arouse great interest among literary historians. Notable among them are:
 * "Ultima Thule" ("far discovery") to honor the new photographic technique; taken in November 1848 in Providence, Rhode Island, probably by Edwin H. Manchester
 * "Annie", given to Poe's friend Annie L. Richmond; probably taken in June 1849 in Lowell, Massachusetts, photographer unknown

Poe Toaster
Between 1949 and 2009, a bottle of cognac and three roses were left at Poe's original grave marker every January 19 by an unknown visitor affectionately referred to as the "Poe Toaster". Sam Porpora was a historian at the Westminster Church in Baltimore, where Poe is buried; he claimed on August 15, 2007, that he had started the tradition in 1949. Porpora said that the tradition began in order to raise money and enhance the profile of the church. His story has not been confirmed, and some details which he gave to the press are factually inaccurate. The Poe Toaster's last appearance was on January 19, 2009, the day of Poe's bicentennial.

List of selected works
Short stories

 * "The Black Cat"
 * "The Cask of Amontillado"
 * "A Descent into the Maelström"
 * "The Facts in the Case of M. Valdemar"
 * "The Fall of the House of Usher"
 * "The Gold-Bug"
 * "Hop-Frog"
 * "The Imp of the Perverse"
 * "Ligeia"
 * "The Masque of the Red Death"
 * "Morella"
 * "The Murders in the Rue Morgue"
 * "Never Bet the Devil Your Head"
 * "The Oval Portrait"
 * "The Pit and the Pendulum"
 * "The Premature Burial"
 * "The Purloined Letter"
 * "The System of Doctor Tarr and Professor Fether"
 * "The Tell-Tale Heart"
 * "Loss of Breath"

Poetry

 * "Al Aaraaf"
 * "Annabel Lee"
 * "The Bells"
 * "The City in the Sea"
 * "The Conqueror Worm"
 * "A Dream Within a Dream"
 * "Eldorado"
 * "Eulalie"
 * "The Haunted Palace"
 * "To Helen"
 * "Lenore"
 * "Tamerlane"
 * "The Raven"
 * "Ulalume"

Other works
 * Politian (1835) – Poe's only play
 * The Narrative of Arthur Gordon Pym of Nantucket (1838) – Poe's only complete novel
 * The Journal of Julius Rodman (1840) – Poe's second, unfinished novel
 * "The Balloon-Hoax" (1844) – A journalistic hoax printed as a true story
 * "The Philosophy of Composition" (1846) – Essay
 * Eureka: A Prose Poem (1848) – Essay
 * "The Poetic Principle" (1848) – Essay
 * "The Light-House" (1849) – Poe's last, incomplete work

title: Edgar Allan Poe