Jiaqi Nie

S Shyam Sundar

Who Would Pay for Facebook? Self Esteem as a Predictor of User Behavior, Identity Construction and Valuation of Virtual Possessions

Keywords: self-esteem, Facebook activity, online identity construction, virtual possession, survey, quantitative methods, structural equation modeling (SEM)

Self-presentation is a major preoccupation in Facebook. Users carefully construct their online profile and assiduously edit postings on their wall in order to strategically shape their online persona. This study examines some psychological antecedents and consequences of such actions. In particular, we propose that users' self-esteem affects their sense of agency and self-monitoring tendencies, with the former leading to a fuller description of their profile and the latter contributing to more frequent customization of their wall. In turn, these behaviors are hypothesized to contribute to users' personal and social identity respectively, en route to affecting their valuation of Facebook as a virtual possession. Structural equation modeling analysis of survey data (N=221) largely supports this model and reveals that the personal identity reflected in one's Facebook account is a major predictor of the degree to which one values it as a possession. We discuss the implications of "I" vs. "Me" in self-esteem with regard to virtual possessions in social networking environments.

Introduction

It is natural for humans to view the self as being situated in a social experience. Individuals construct their identity by carefully articulating their personality and paying close attention to how they present themselves to others. With the arrival of networked media, this articulation becomes even more pronounced. Internet users, particularly those who have social media accounts, must now consider their online personas in addition to-or as part of-their real-life identities. It has been well recognized by scholars that the Internet is challenging and shaping one's identity [START_REF] Greenhow | Informal Learning and Identity Formation in Online Social Networks[END_REF][START_REF] Sundar | Self as Source: Agency and Customization in Interactive Media[END_REF][START_REF] Turkle | Life on the Screen: Identity in the Age of the Internet[END_REF]. In fact, many studies have identified effects of Internet use on individual identity. Research has focused on the changes in self-concept ushered in by the Internet, giving rise to the notion of "second self" as an online extension of one's psychological and social self [START_REF] Turkle | The Second Self: Computers and the Human Spirit[END_REF], leading to a conscious accumulation of "virtual possessions" [START_REF] Odom | Virtual possessions[END_REF], such as digital images, music files and even the number of contacts on one's social networking site. All this suggests that while the psychology of one's offline self may predict our online activities related to shaping our online self, these activities in turn would likely dictate the degree to which we define ourselves based on our online self and the extent to which we value our virtual possessions. An understanding of the psychological mechanisms involved in the translation of one's self concept to one's online identity can be quite insightful in enhancing our theoretical understanding of user motivations for specific activities in social media sites. It also has practical implications for the valuation of such sites (e.g., how much users of social networking sites would be willing to pay for the service if it were not free) and the design of identity-enhancing features that would maximize the value of virtual possessions for users.

We begin with a review of the theoretical framework underlying self-concept by focusing on the two facets of "Me" and "I" based on Mead's theory, and propose a series of hypotheses involving psychological concepts as well as user behaviors and attitudes pertaining to Facebook, the popular social networking site [START_REF] Mead | Mind, self, and society: From the Standpoint of a Social Behaviorist[END_REF]. The model emerging from these hypotheses is then tested using structural equation modeling analysis, followed by a discussion of the specific associations discovered in the study and their implications for theory and practice.

2

Literature Review

Mead's "I" & "Me" in Self-esteem

Several decades before the advent of social media, G.H. Mead [START_REF] Kolb | A Critical Evaluation of Mead's "I" and[END_REF] developed a systematic theory of the social nature of self by introducing the concepts of the "I" and "Me." According to this theory, individuals construct the "I" and the "Me" based on personal selfhood [START_REF] Mead | Mind, self, and society: From the Standpoint of a Social Behaviorist[END_REF]. From a perspective of the world which can be reinterpreted by our actions and the feedback we receive, there are, on the one side, individuals who care about the "I" in selfhood, trying to know themselves by observing their own behaviors; on the other hand, the "Me" in one's self-concept is focused on what is learned in the interaction with others, and more generally with the environment. Individuals' specific views about selfhood and the way they frame these views identify and contribute to their global self-esteem [START_REF] Pelham | From Self-conceptions to Self-worth: on the Sources and Structure of Global Self-esteem[END_REF]. Individuals who see positive self-views from their own behavior are especially likely to be high in self-esteem. On the other hand, individuals who derive self-views from their interactions with others and the environment tend to be low in self-esteem.

Self-esteem lies at the core of one's self-concept and serves as the starting point for our examination of user identity in social media. Insofar as this study is concerned, 'self-esteem' refers to the evaluation of oneself, which is conceived both as a trait and a state of being [START_REF] Crocker | Contingencies of Self-worth[END_REF]. We adopt the term 'self-esteem' to depict an individual's selfhood from a broad perspective, as proposed by Campbell et al. [START_REF] Campbell | Selfconcept Clarity: Measurement, Personality Correlates, and Cultural Boundaries[END_REF]. People with high self-esteem are consistent and clear about themselves, with the "I" dominating in one's persona; whereas people with low self-esteem tend to be confused about their self concept and feel anxious about self-relevant images in public; their character is strongly influenced and controlled by "Me" rather than "I" [START_REF] Baumeister | Does High Self-Esteem Cause Better Performance, Interpersonal Success, Happiness, or Healthier Lifestyles?[END_REF][START_REF] Bem | Self-perception Theory[END_REF][START_REF] Tice | Self-concept Change and Self-presentation: the Looking Glass Self is also a Magnifying Glass[END_REF].

More broadly, the twin aspects of "I" and "Me," generated by high and low selfesteem respectively, are likely to lead to two different paths in influencing our online attitudes and behaviors, shaping our online identity, and eventually changing our valuation of our virtual possessions, as discussed in the sections that follow.

Psychological Factors and Behaviors Based on Self-esteem

There is consensus among scholars that self-esteem shapes the way we perceive ourselves and the strategies we use to present ourselves. On the one hand, self-esteem could be a proxy for the psychological sense of agency. Individuals who score high on self-esteem have a higher sense of self-control and enhanced initiative [START_REF] Baumeister | Does High Self-Esteem Cause Better Performance, Interpersonal Success, Happiness, or Healthier Lifestyles?[END_REF]. Higher self-esteem refers to a more favorable evaluation of oneself, which has profound consequences for one's "sense of agency" [START_REF] Judge | Are Measures of Self-esteem, Neuroticism, Locus of Control, and Generalized Self-efficacy Indicators of a Common Core Construct?[END_REF]. The term "sense of agency" is used to describe the "sense that I am the one who is causing or generating an action" ( [START_REF] Gallagher | Philosophical Conceptions of the Self: Implications for Cognitive Science[END_REF], p.15). Therefore, we hypothesize: H1: The higher the level of self-esteem, the higher the degree of sense of agency.

On the other hand, when it is low, self-esteem is related to self-monitoring in public presentations. The need for impression management and self-monitoring is critical when those with low self-esteem express themselves publicly in SNS [START_REF] Zarghooni | A Study of Self-Presentation in Light of Facebook[END_REF]. Empirically, social media researchers adopt self-esteem, an overall affective psychosocial development measure, in evaluating one's self or personality, to observe virtual selfhood in social networking sites like Facebook. For example, individuals with lower self-esteem have been shown to experience a higher level of bridging social capital by increasing their intensity of Facebook usage, measured in terms of the customization behaviors on one's Facebook wall [START_REF] Steinfield | Social Capital, Self-esteem, and Use of Online Social Network Sites: a Longitudinal Analysis[END_REF]. This demonstrates the "Me" aspect of selfconcept, a tendency to monitor oneself constantly and rely on others for boosting one's self-esteem. Thus, we propose that: H2: The lower the level of self-esteem, the higher the level of self-monitoring. According to Mead's social behaviorism [START_REF] Mead | Mind, self, and society: From the Standpoint of a Social Behaviorist[END_REF], "individuals become aware of and learn to guide their behavior" during their process in defining and studying the idea of the self (as cited in [START_REF] Manis | Symbolic Interaction: a Reader in Social Psychology[END_REF], p. 165). In other words, individuals adjust their behaviors and shape their images available to others based on their view of themselves [START_REF] Goffman | The Presentation of Self to Others[END_REF]. Facebook enables its users to communicate with a diverse set of affordances ("action possibilities"), such as posting comments, sharing pictures, and liking or highlighting contents [START_REF] Ellison | Facebook Use on Campus: A Social Capital Perspective on Social Network Sites[END_REF]. This is specifically true after Facebook released its timeline version, which offers users more self-monitoring options on their Facebook wall. Therefore, we can expect users with low levels of self-esteem to be overly conscious of their wall and proceed to use the available customization options, such as editing certain pictures, removing tags and deleting comments. On the other hand, the profile space in Facebook is a key location for self-presentation that can help boost one's sense of agency. Online SNS profiles are the epitome of Web-enabled authorship wherein they can exercise enormous agency. The completion of the Facebook profile, insofar as defining one's presence on social networks, indicates how much individuals want to construct one's full identity on Facebook. Previous studies have found that individuals report higher self-esteem when they make changes to their online profiles, because of heightened self-awareness and self-presentation [START_REF] Gonzales | Mirror, Mirror on My Facebook Wall: Effects of Exposure to Facebook on Self-esteem[END_REF].

Based on the preceding review of theoretical and empirical work, we propose the following hypotheses for study in the SNS context: H3: The higher the sense of agency for individuals, the higher the level of profile completion on Facebook.

H4: The higher the level of self-monitoring, the greater the frequency of wall customization in Facebook.

Facebook as Personal Identity vs. Social Identity

Facebook affords the ability to portray oneself in the best possible light. By either monitoring the Facebook wall or customizing the presentation on Facebook, users can carefully mold one's online identity. However, no matter how changeable an identity could be in Facebook, the original characteristic in oneself would still be the determining factor in one's online identity. In other words, although users of Facebook tend to create certain forms of online identity for themselves and are allowed more liberty to shape it than in the offline world, they still operate within the constraints of their original offline identity. The relationship between online identity and offline identity is bidirectional and mediated by a host of psychological factors and behaviors.

In general, identity, conceptualized by previous researchers as "an individual's perception of selfhood," contains two basic facets [START_REF] Bong | Comparison between Self-concept and Self-efficacy in Academic Motivation Research[END_REF][START_REF] Byrne | The General/Academic Self-concept Nomological Network: A Review of Construct Validation Research[END_REF]. One focuses on individuals' self-consciousness and self-reflection. The other facet, labeled "social identity," goes one step further by paying attention to personal behaviors within a group or public context [START_REF] Reynolds | Interactionism in Personality and Social Psychology : An Integrated Approach to Understanding the Mind and Behaviour[END_REF]. Different from the personal identity, which is based on personal accomplishment and material attainment, social identity becomes a matter of impression management, directed at motivating others to achieve and maintain positive concepts of oneself in public [START_REF] Gecas | Self and Identity[END_REF][START_REF] Hogg | Social Identity, Self-Categorization, and the Communication of Group Norms[END_REF].

Empirical studies have confirmed the direct relationships between different types of Facebook user behaviors and their identity. DiMicco and Millen [START_REF] Dimicco | Identity Management: Multiple Presentations of Self in Facebook[END_REF] conducted a survey based on the Facebook profiles visible when browsing IBM's network to examine the relationship between a Facebook users' profile behaviors and their online identities. They found that individuals use different profile pages in order to maintain multiple self-identities on Facebook. Ellison et al. [START_REF] Ellison | The benefits of Facebook "friends": Social capital and college students' use of online social network sites[END_REF] showed that people with low self-esteem gained more social capital, which can strengthen their social identity through Facebook if they update their wall more intensely. Basically, what individuals "invest" in "who they are" allows them to reconfirm the personal identity facet inside one's selfhood; while what they "invest" in "who they are communicating with" leads them to awaken the social identity component inside them.

Thus, under this line of reasoning, it is to be expected that identity, perceived by individuals within the context of Facebook, will also contain two dimensions deter-mined by selfhood and self-esteem. Specifically, we propose that the "I"-dominated high self-esteem will connect to personal identity while the "Me"-influenced low selfesteem will relate to social identity. Based on this rationale, we hypothesize, H5. The higher the level of profile completion on Facebook, the higher the degree of perceived personal identity from Facebook.

H6. The greater the frequency with which individuals customize their Facebook wall, the higher their degree of perceived social identity from Facebook.

Identity and Virtual Possession

If indeed our online activities can shape our identity, the products of those activities (the conversation threads archived, the digital images tagged, and so on) are likely to become treasured "virtual possessions". Given the increasing amount of time and resources spent on social networking and other online activities, we are entering the realm of "real virtuality," wherein the mediated world is a major aspect of our everyday "reality" [START_REF] Turkle | Alone Together: Why We Expect More from Technology and Less from Each Other[END_REF]. The boundary between material possessions and virtual possessions is now blurring. Odom and his colleagues define virtual possessions as "artifacts that are increasingly becoming intangible (i.e. books, photos, music) and things that have never been traditionally considered as tangible goods, (i.e., SMS archives, social networking profiles)" ( [START_REF] Odom | Teenagers and their Virtual Possessions : Design Opportunities and Issues[END_REF], p. 1491). Facebook provides individuals with specific, virtual information wherein users benefit from a sense of oneself. In the present study, we suggest that Facebook acts as a virtual container of online social information (about oneself) that may be considered as "virtual possessions" by its users. Given this, it is necessary to analyze the factors that impact the role of one's own network on Facebook as a virtual possession and how we value this possession.

Any possession is based on attachment with certain specific meanings. As Odom et al. note, a possession should have its value in either fulfilling one's life or helping establish an idealized future goal [START_REF] Odom | Teenagers and their Virtual Possessions : Design Opportunities and Issues[END_REF][START_REF] Csikszentmihalyi | The Meaning of Things: Domestic Symbols and the Self[END_REF][START_REF] Kleine | An Integrative Review of Material Possession Attachment[END_REF]. Researchers argue that individuals form a meaningful attachment toward possessions through the process of self-extension [START_REF] Belk | Possessions and the Extended Self[END_REF]. Therein, lessons learned about possession-attachment develop and form a template (internal working model) on which relationships are built for life, which is relatively close to self-concept and self-identity. By mapping these mechanisms from material possession constructs, the following hypotheses can be posited about the relationship between online identity and virtual possessions.

H7: The higher the degree of personal identity perceived from Facebook by individuals, the higher the value they place on Facebook as a virtual possession.

H8: The higher the degree of social identity perceived from Facebook by individuals, the higher the value they place on Facebook as a virtual possession.

Taken together, a hypothetical structural model is proposed to predict how psychological differences of Facebook users affects their valuation of virtual possessions by examining different types of Facebook user behaviors, leading to different attitude outcomes pertaining to their personal and social identity (See Fig. 1). A person's selfesteem is expected to positively predict their sense of agency, which in turn is associated with more Facebook profile behavior and a higher degree of personal identity perceived in Facebook. On the other hand, self-esteem is expected to negatively predict the level of self-monitoring, which is associated with more Facebook wall customization and a higher degree of social identity perceived in Facebook. Both personal identity and social identity derived from Facebook are said to predict the value that users place on Facebook as a virtual possession. On the first page of the study, participants were informed that they were participating in an investigation of individuals' Facebook usage. All participants were offered an informed consent form and a pre-knowledge question prior to their participation. Respondents were given the sample picture of the two types of Facebook interfaceoriginal interface and timeline interface. They were asked to choose the Facebook interface that they are currently using. Upon consenting with a click, they were directed to the corresponding questionnaire.

Before launch of the study, we pretested the entire questionnaire with 24 students (14 females and 10 males) from the same Korean university, who were also paid 2000 won for their participation. Based on the insights gained from the pretest, we were able to prepare the final version of the questionnaire by modifying and adding some items, especially questions about Facebook use behaviors.

Measurement

According to Facebook user menu and pretest results, we operationalized user behaviors on Facebook profile as "the degree of completing the profile fields" and Facebook wall behaviors as "the frequency with which users customized their Facebook wall." Respondents were asked to log into their Facebook account and answer the questions based on what they saw on their Facebook profile and wall. In the Facebook profile section, 33 categories were listed for measuring their profile behavior, such as "work", "relationship", "family", etc. Participants were asked to answer whether they have filled in those fields or not by selecting "yes" or "no". The number of categories that were indicated as "yes" was summed up as a continuous measure for "Facebook Profile Behavior" (M = 12.72, SD = 6.58). For the Facebook wall section, due to different functions provided in the timeline interface and the original interface, we chose the common items across the two interfaces. Specifically the customizable items are "change friend list", "delete posts", "change date", and "add position". The frequency for the four items was measured by using 0-7 scale (0 = Never, 7 = daily). The mean value of the four items was calculated as the value for Facebook wall behavior (M = 2.69, SD = 1.31).

In order to measure individual's level of self-esteem, six items were adopted from the Rosenberg Self-Esteem Scale [START_REF] Rosenberg | Society and the Adolescent Self-image[END_REF], including "I feel that I'm a person of worth, at least on an equal plane with others", "I have a bunch of friends that care about me", and so on (Cronbach's α = .81). Responses were reported on a 7-point Likert scale with higher scores indicating higher self-esteem.

Sense of agency was measured via nine items based on Kim & Sundar [START_REF] Kim | Can Your Avatar Improve Your Health? The Impact of Avatar Attractiveness and Avatar Creation[END_REF]. These items consisted of "I feel that I can control my destiny", "I feel like I can exercise my free will", "I feel like I can access information that is appropriate for me", and so on. These items were measured using a 1-7 scale, ranging from strongly disagree to strongly agree (Cronbach's α = .86).

The Self-Monitoring scale has proven useful in measuring individuals' sensitivity to social presentation [START_REF] Berinsky | Can We Talk ? Self-Presentation and the Survey Response[END_REF]. However, given the present study's focus on presenting oneself in the most positive light to others and based on psychometric data about the scale from past studies, six questions were adapted from the Other-Directedness Factor of the Self-Monitoring Scale [START_REF] Briggs | An Analysis of the Self-monitoring Scale[END_REF]. Example items include "In different situations and with different people, I often act like very different persons", "In order to get along and be liked, I tend to be what people expect me to be rather than anything else", and so on. These items were measured using a 1-7 scale, ranging from strongly disagree to strongly agree. The ratings across the six items were averaged to yield the "Self-monitoring" index (Cronbach' α = .75).

We selected four items capturing the degree of public attachment from Ball and Tasaki's nine-item scale and revised them to reflect the participant's relationship with Facebook (e.g., "If someone hacked my Facebook page, I would feel like I have been personally attacked," etc.) [START_REF] Ball | The Role and Measurement of Attachment in Consumer Behavior[END_REF]. The questions loaded under the factor labeled "Facebook as Social identity" (Cronbach's α = .68).

As we suggested, the emotional significance of one's Facebook profile would strengthen the personal facet of attachment. Therefore, an amended version of the Possession-Self link Scale [START_REF] Ferraro | Our Possessions, Our Selves: Domains of Selfworth and the Possession-self Link[END_REF] was adopted to measure the personal identity derived from Facebook. The answers to these questions were recorded on a 7-point Likert scale with a higher score indicating greater degree of perceived personal identity from Facebook. Reliability among the five items (e.g., "Facebook has helped shape my identity", "Facebook has helped me narrow the gap between what I am and what I try to be", etc.) was high (Cronbach's α = .90).

To yield the Virtual Possession index, five questions were created to capture the degree to which participants value Facebook. The monetary value relating to a social networking site as virtual possession was measured by the following items: "I don't care if all my data on Facebook is transferred to another website (Reverse)", "I would like to pay to add music on my Facebook", "If Facebook starts selling Facebook Emoticons, I am willing to pay for it", "If Facebook starts selling Facebook Wall paper, I am willing to pay for it", and "If Facebook starts selling Facebook Wall themes I am willing to pay for it" (Cronbach's α = .71).

In order to control for potentially confounding factors, individual demographic questions were asked (e.g., age, gender, grade, etc.). The Facebook Use Intensity Scale [START_REF] Ellison | The benefits of Facebook "friends": Social capital and college students' use of online social network sites[END_REF] was also used to control individual differences in Facebook use behavior as well as virtual possession attachment (e.g., "Facebook is part of my everyday activity", "Facebook has become part of my daily routine"). The six questions were asked on a 1-7 scale, with 1 representing strongly disagree and 7 representing strongly agree (Cronbach's α = .80).

Model Specification

Structural equation modeling (SEM) using AMOS 18.0 was used to test the hypothesized model. The hypothesized model estimated the extent to which individual differences on self-esteem predict their values on Facebook as a virtual possession, by identifying two different paths based on the theoretical discussion of the psychological differences of self concept on "Me" versus "I"-focused psychological characteristics and behaviors. One path was concerned with the concepts of sense of agency and Facebook profile behavior while the other one pertained to self monitoring and Facebook wall customization activity (see Fig. 1). Self-esteem was entered as the lone exogenous variable, with the two psychological variables (sense of agency, selfmonitoring), Facebook use behaviors (profile behavior, wall behavior) and their attitude outcomes (Facebook as personal identity, Facebook as social identity), as well as the valuation of Facebook as a virtual possession, serving as endogenous variables.

Model Fit Indices

This study followed the indicators provided in AMOS to assess the overall model fit, including chi-square (χ2), the root mean square error of approximation (RMSEA), the goodness of fit index (GFI), the standardized root mean square residual (SRMR) and comparative fit index (CFI). Considering the fact that the chi-square test is sensitive to sample size [START_REF] Bentler | Comparative Fit Indexes in Structural Models[END_REF], the other fit indices were consulted for determining the degree to which the data fit the hypothesized model.

Results

Preliminary Analysis

The data were checked for normality and outliers before the analyses. Based on the Mahalanobis distance criterion, four cases were identified as multivariate outliers and therefore removed from the dataset. As a result, the final sample size was 221. Using Kline's guideline for univariate normality, where an absolute value of less than 3.0 for skewness and 10.0 for kurtosis are considered normal, all variables in the model were normal [START_REF] Kline | Principles and Practice of Structural Equation Modeling[END_REF]. In addition, multivariate normality was checked. The criterion for multivariate normality is that the Mardia's coefficient should be lower than p (p+2), where p is the number of observed variables [START_REF] Bollen | Structural Equation Models[END_REF]. The Mardia's coefficient for the current study was 150.06. Given that we had 25 observed variables, we can conclude multivariate normality.

Table 1. Correlations and descriptive statistics for all measured variables

Before fitting the hypothesized model, a confirmatory factor analysis (CFA) was performed to verify the factor structure of the observed variables in the model. The model fit was poor according to the following indices in CFA: χ2 = 1782.405, df = 838, p < .001; RMSEA = .072 (CI = .067-.076); CFI = .837; GFI = .716. Two items from 'Self-esteem', five items from 'Sense of Agency' and two items from 'Selfmonitoring' variable were iteratively removed from the dataset because they were not significantly correlated with and/or had low factor loadings on their respective latent constructs. After eliminating these items, fit indices improved: χ2 = 699.507, df = 384, p < .001; RMSEA = .061 (CI = .054 -.068); CFI = .918; GFI = .825. Furthermore, modification indices suggested allowing the following three pairs of error terms from 'Facebook as Personal Identity' and a pair of error terms from 'Self-esteem' and 'sense of agency' to covary: (1) "Facebook has helped shape my identity" with "Facebook has helped me narrow the gap between what I am and what I try to be"; (2) "Facebook has helped me narrow the gap between what I am and what I try to be" with "My Facebook page is part of who I am"; (3) "My Facebook page is part of who I am" with "I derive some of my personal identity from my Facebook page"; (4) "I have a distinct identity" with "I feel that I can control my destiny"; (5) "I take a positive attitude toward myself" with "On the whole, I am satisfied with myself". Finally, the revised model had an acceptable fit: χ2 = 363.999, df = 255, p < .001; RMSEA = .044 (CI = .033 -.054); CFI = .965; GFI = .885.

Based on the result of the CFA, reliability for each construct was evaluated with the remaining items. In order to test the interrelationships among the constructs the model, the reliability (Cronbach's alpha) and the zero-order correlations were computed (see Table 1). Most of these bivariate correlations were significant and in a direction that was consistent with the hypotheses.

Testing the Hypothesized Model

To test the hypothesized model, SEM was conducted with the maximum likelihood estimation method. The result showed that the overall model was a reasonable fit in terms of RMSEA and CFI, but not SRMR and GFI: χ2 = 591.125, df = 314, p < .001; RMSEA = .063 (90% CI: .055 -.071); CFI = .912; SRMR = .1166; GFI = .842. Therefore, model modifications were conducted based on theoretical rationale and modification indices. First, a pathway from 'Facebook wall behavior' to 'Facebook as Social Identity (FASI)' (β = 0.085, p = .288) was detected as non-significant. Owing to the fact that the study failed to support this originally hypothesized path, and considering the fact that 'FASI' holds no theoretical value as an independent exogenous variable, this latent construct was removed from the model entirely. After the deletion of 'FASI', overall model fit improved: χ2 = 400.537, df =244, p < .001; RMSEA = .054 (90% CI: .044 -.063); CFI = .947; SRMR = .0852; GFI = .869. Yet, the fit of this model hovered around acceptable criteria.

After considering the modification indices and based on theoretical rationale, a path from 'Facebook wall behavior' to 'Facebook profile behavior' was added to the model. The change resulted in the following fit statistics: χ2 = 370.213, df = 243, p < .001; RMSEA = .049 (90% CI: .039 -.059); CFI = .957; SRMR = .0782; GFI = .877. Since 'FASI' was removed, the direct path from 'Facebook wall behavior' to 'Facebook as Personal Identity (FAPI)' was suggested by the modification indices. The data suggest that the more Facebook users engage in customization activities on the wall, the greater the sense of personal identity perceived from Facebook, instead of a sense of social identity. From a theoretical perspective, customization of the wall is likely to be associated with personal identity according to the agency model of customization [START_REF] Sundar | Self as Source: Agency and Customization in Interactive Media[END_REF]. After adding the path from 'Facebook wall behavior' to 'FAPI', the overall model fit improved to a respectable level: χ2 = 361.897, df = 242, p < .001; RMSEA = .047 (90% CI: .037 -.057); CFI = .959; SRMR = .0729; GFI = .880. Fig. 2 depicts the final model that was retained for interpretation.

Fig. 2. Standardized path coefficients for the final model

Summary of Findings

The analysis showed that self-esteem has a significant positive effect on sense of agency (β = .82, p < .001; H1) and a significant negative effect on self-monitoring (β = -.24, p < .005; H2), meaning that individuals with a higher self-esteem have a higher sense of agency, while individuals who have a lower self-esteem tend to use self-monitoring strategy to control how they present themselves in the public (H1, H2 were supported). The results also suggest that self-esteem had stronger direct effects on sense of agency than on self-monitoring.

In addition, sense of agency is associated significantly with the information that was inputted in one's profile (β = .14, p < .05; H3); and the relationship between selfmonitoring and Facebook wall behavior was also statistically significant (β = .17, p < .05; H4) (H3, H4 were supported). Although the variances explained by each psychological factor (sense of agency, self-monitoring) on the respective Facebook behaviors are not large, they are meaningful enough to reveal the relationships between selfesteem and the different kinds of Facebook activities by users. Furthermore, the path from Facebook wall behavior to Facebook profile behavior showed a significant relationship (β = .36, p < .001). This means that if individuals customize their Facebook wall more frequently to promote themselves in public, they are more likely to input more information in their profiles and thereby establish a more comprehensive online presence.

More interestingly, both the profile behavior and wall behavior are significantly associated with Facebook as Personal Identity (FAPI). Thus, both the amount of information in their profile (β = .17, p < .05) and the degree of wall customization (β = .21, p < .05) are positively related to the sense of personal identity derived from Facebook (H5 was supported) rather than social identity (H6 was rejected). Ten percent of the variance for perceived personal identity from Facebook is accounted for by these two behaviors.

Also, results show that the higher the degree of personal identity perceived from Facebook, the higher the value of Facebook as a virtual possession for the users (β = .60, p < .001). Therefore, H7 was supported. The estimated variance of virtual possession accounted for by FAPI is 0.36. In sum, the data show that self-esteem is positively related to sense of agency and negatively related to self-monitoring, with these psychological characteristics predicting user behavior on Facebook. We found that the more users engage in using Facebook, both for completing Facebook profile and customizing Facebook wall, the more they attached their personal identities to Facebook because both the profile and the wall end up reflecting the self. Table 2 provides an overview of the estimates, standard errors and critical ratios for the paths in the model.

Discussion

Overall, our analysis suggests, based on structural equation modeling, that the psychological factors underlying Facebook user behaviors influence the degree to which Facebook is seen as a reflection of one's personal identity and the extent to which this social-media site is valued as a virtual possession.

The "I" vs. "Me" Paradigm in Facebook

This study proposed two possible paradigms, the "I" and the "Me", in one's selfhood within the virtual reality of SNS. As we hypothesized, self-esteem is related to the two facets of selfhood, each associated with distinct psychological factors and Facebook behaviors. The sense of "I"-ness focuses on a reflection of true inner self; the sense of "Me"-ness is externally influenced, wherein one's sense of self is derived For those individuals who have the "I" oriented personality, i.e., a higher level of self-esteem, a higher sense of agency was reported; while for those individuals with a "Me" orientation and a lower level of self-esteem, a higher need for self-monitoring was noted. These psychological states are able to predict distinct user behaviors on Facebook. Individuals who perceived a higher sense of agency tend to complete their profiles with more information, showing their true self and constructing a detailed online presence. On the other hand, individuals who show a higher level of self-monitoring tend to customize their online public presentations (by altering entries in their Facebook wall) more frequently. These behaviors have implications for the design of audience awareness mechanisms. On the one hand, interface features could be designed to inform users about the number of times their profile and wall postings were viewed by others. On the other hand, a system utility can be designed to keep track of the frequency of changes to the user's profile and modifications of wall postings.

As it turns out, these two activities are related. We found that individuals who customize their Facebook wall more frequently have a higher level of profile completion. It suggests that the degree to which individuals promote themselves in public has an influence on their desire to construct a formal and complete online presence. In turn, profile behavior emerged as an important construct connecting one's self-esteem to the tendency for projecting one's online personal identity through Facebook in the final model. All this implies that users value the profile space in Facebook as an important part of their online persona construction. Therefore, design changes to Facebook profile should be geared toward affording users the ability to customize in ways that can project their personal identity.

One of the notable findings of this study is that instead of increasing a sense of social identity as hypothesized, behavior on the Facebook wall (that is presumably meant for carefully articulating one's public self-presentation) was actually positively correlated with private identity-focused attachment. While some previous research has shown that the other-directedness in self-monitoring is negatively correlated with self-focused identity building, other research has shown that self-monitoring could also be seen as self-expression rather than self-promotion [START_REF] Lee | Psychological Effects of Culture on Aesthetic Motivation for Cosmetic Customization of Mobile Phones[END_REF]. Instead of simply advocating oneself, self-monitors may be more likely to use this strategy to construct a self-concentrated identity [START_REF] Snyder | Self-monitoring Processes in Organizational Settings[END_REF]. Our data suggest that, over time, users accept Facebook as part of their routine and do not see it simply as a networking tool, but rather as a venue for expressing and shaping one's self identity, be it their true self or artificial self. This has implications for Web 2.0 application design, in that greater emphasis should be placed on tools that help users shape their personal identity. Based on their online activities, individual users could be offered a personalized palette of online personas to choose from so that they can visualize how others may view them. Various framing strategies can be offered to users with the help of tools that ask users whether they want to be seen by others in this light or that way, and then proceed to identify those profile elements and wall activities that are consistent and those that are inconsistent with their chosen identity, so that users can make judgments about their own online identity as well as decisions about projecting the type of identity that they wish to present to others.

Personal Identity and Virtual Possession

A statistically significant portion of the variance in perceived attachment to Facebook as personal identity can be explained by the two kinds of behaviors we examined in this study. The results indicate that Facebook successfully attracts users who create different strategies for representing themselves online and, more importantly, for expressing their true selves. As mentioned above, the present study only distinguishes two types of user behaviors on Facebook -information completion in the profile section and customization frequency in the wall section. While there are clearly many other behaviors in Facebook, we could only include those that lend themselves to counting and are explicitly recorded on Facebook, given the limitations of the selfreport survey method that we employed. Hence, it is not surprising that the variance in self-focused attachment to Facebook that is explained by the two kinds of behaviors measured in the current study is only 10 percent, but it is nevertheless a significant indicator of the behavior-attachment connection.

Even though we focused on Facebook's potential to promote both social identity and personal identity as potential predictors of users' valuation of Facebook as a virtual possession, it was clear from the data that Facebook as Personal Identity played the more important role in the model. This demonstrates how closely linked Facebook is to one's representation of self-identity and how it affects the extent to which one values Facebook. This effect differs from the public-focused self-identity that users perceive from Facebook, which failed to show a significant association with Facebook user behaviors measured in this study. In fact, compared to the frequency of wall customizing, the extent of profile completion has a greater influence on the value of Facebook as a virtual possession, through personal identity. This has important implications, as discussed in the next section.

Value System of Virtual Possessions

Ultimately, it appears that self-esteem is a driving force behind our SNS use, and the extent to which the SNS reflects our personal identity is indeed predictive of the extent to which we value it as a possession. If it reflects our personal identity, we are willing to pay money for purchasing accessories on Facebook. By extension, users who see a strong personal identity reflected in their Facebook environment are more likely to pay for Facebook as a service if it were to charge usage fees. This insight can be quite useful for designers of social media sites and social apps for mobile devices.

To the extent these sites and apps provide affordances for users to construct and shape their personal identity, they are likely to be valued highly by users.

Implications for Web 2.0 Applications Design and Testing

In addition to the aforementioned design implications, our study offers insight into measuring user experience. While traditional HCI literature informs us how to evaluate the quality and usability of websites and applications with usability testing (assessing such constructs as ease of use, usefulness, and navigability), our study identi-fies latent attributes that affect users' loyalty and perceived value of these sites. Our results and discussion clearly show that personal identity is a significant context of use for Web 2.0, meriting greater attention from designers. The strong connection between personal identity and value of Facebook as a virtual possession indicates that the realization of personal identity should be among the quality attributes used for testing. As Orehovački [START_REF] Orehovački | Development of a Methodology for Evaluating the Quality in Use of Web 2.0 Applications[END_REF] suggested, the methodology for evaluating the quality of social networking applications like Facebook should be extended from usability to user experience. Based on the results of our study, we recommend that personal identity, as a subjective quality, be reflected in user-experience metrics when evaluating Web 2.0 applications for design improvements.

Limitations and Directions for Future Studies

This study is not without the usual limitations of exploratory research. First of all, because this study examines the value placed on virtual possessions, with some of them in monetary terms, a more heterogeneous sample -rather than the convenient college group used-is needed to predict users' valuation of SNS as a virtual possession. Second, this survey-based study also suffered from specific disadvantages due to the self-reported responses, especially for questions about specific behavior frequency and situations. Participants may have forgotten pertinent details. Furthermore, as mentioned above, this study did not capture all the categories of Facebook behaviors. It would certainly be interesting to study other categories of Facebook user behaviors such as frequency of profile photo updating and type of privacy setting. In addition, the current study only examined variables in the context of Facebook. For further studies, different SNS services could be used for testing the theoretical model. Despite these limitations, the current study marks an important step in developing a psychological model about self-esteem and self-identity that involves both the antecedents and the consequences of Facebook user behaviors. Implications for SNS owners and advertisers may be derived from the results. For example, designers could consider all the ways in which they can enable users to build their true selves on SNS because a strong personal identity can result in greater attachment and valuation. Advertisers and social media marketers who can leverage the affordances of Facebook to offer users novel and creative opportunities to construct and exhibit their personal identity are likely to be valued more highly than their counterparts who simply focus on exposing Facebook users to their commercial appeals. Likewise, SNS groups that appeal to users' personal identities are likely to garner more support and following, especially when these groups appear as an integral part of one's profile on Facebook.

Conclusion

Social networking technology has opened up numerous possibilities for ideal selfpresentation, which not only helps us realize our individual identity but also shapes our public persona. On Facebook, we customize our wall from time to time by deleting unwanted posts or choosing to experience different levels of interpersonal rela-tionships with different individuals. As shown by our second (and final well-fitting) model, the lower one's self-esteem, the higher the possibility that one will adopt the self-monitoring strategy of camouflaging one's true self. However, our data also suggest that these strategies are not simply adopted to create a perfect social identity, but to articulate one's own ideal self-identity [START_REF] Bargh | The Internet and Social Life[END_REF]. While some scholars (e.g., [START_REF] Turkle | Alone Together: Why We Expect More from Technology and Less from Each Other[END_REF]) argue that technology is diminishing our true selves by encouraging us to constantly seek attention from others, our data suggest that College-age users view SNS technology as an extension of oneself, not a substitute. Individuals still possess a strong eagerness to define their individual identity and to control their own feelings in a social networking environment, especially when they have a high sense of self-esteem. The eagerness to have a full online presence seems to be stronger than the desire to monitor oneself in public social networks. Further, it is clear that self-esteem affects the value we place on Facebook as a virtual possession largely because we see Facebook as a vehicle for shaping and representing our personal identity. In conclusion, our valuation of Facebook as a virtual possession is largely a result of the "I" dominated selfhood rather than a "Me" dominated outlook. We value Facebook and spend so much time with it because it offers us numerous opportunities to construct, shape and reflect our personal identity, not because it allows us to project a certain social identity to members of our network. Therefore, despite all its socialness, Facebook is a deeply personal medium.

Fig. 1

 1 Fig. 1. Hypothesized Model

Table 2 .

 2 Estimated (Standardized & Unstandardized) S.E. and C.R. for the supported hypothesized paths.

  All of the paths were statistically significant at p < .05

	Paths	Estimates (Unstandardized)	Estimates (standardized)	Standard Error	Critical Ratio
	Self-monitoring <---Self-esteem	-0.28	-0.24	0.10	-2.90
	FB WB<---Self-monitoring	0.26	0.17	0.11	2.27
	Sense of Agency <---Self-esteem	0.84	0.83	0.13	6.64
	FB PB <---Sense of Agency	0.17	0.14	0.09	1.95
	FB PB<---FB WB	0.25	0.36	0.04	5.72
	FAPI <---FB PB	0.23	0.17	0.98	2.33
	FAPI <---FB WB	0.20	0.21	0.70	2.92
	Virtual Possession <---FAPI	0.68	0.60	0.74	9.21
	Note:

FAPI: Facebook as Personal Identity; FB: Facebook; WB: Wall Behavior; PB: Profile Behavior from one's public presentation.

Acknowledgement

This research is supported by the Singapore National Research Foundation under its International Research Center Keio-NUS CUTE Center @ Singapore Funding Initiative and administered by the IDM Program Office.


Katharina Gerken
email: katharina.b.gerken@student.hs-rm.de

Sven Frechenhäuser

Ralf Dörner

Johannes Luderschmidt
email: johannes.luderschmidt@hs-rm.de

Authoring Support for Post-WIMP Applications

Keywords: authoring processes, authoring tools, post-WIMP interfaces, cross layer

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Authoring processes and tools for creating classical GUIs (Graphical User Interfaces) that adhere to the WIMP (Windows, Icons, Menus, Pointer) paradigm have been explored and refined over the years and developers have been honing their skills in creating these types of user interfaces. With post-WIMP [START_REF] Van Dam | Post-WIMP User Interfaces[END_REF] becoming more commonplace [START_REF] Lawson | Component-Based High Fidelity Interactive Prototyping of Post-WIMP Interactions[END_REF], the authoring process for user interfaces is becoming more complex, as the diversity of interaction methods rises [START_REF] Bastide | A: Model-Based Tool for Interactive Prototyping of Highly Interactive Applications[END_REF][START_REF] Beaudouin-Lafon | Instrumental Interaction: an Interaction Model for Designing Post-WIMP User Interfaces[END_REF]. Novel tasks need to be carried out during authoring and new skill sets are required, for example expertise in reality-based interaction [START_REF] Jacob | Reality-Based Interaction: a Framework for Post-WIMP Interfaces[END_REF], in order to develop post-WIMP user interfaces. As single authors seldom possess the whole range of skills required, collaboration of multiple author groups with specific expert knowledge is often needed [START_REF] Huot | The MaggLite Post-WIMP Toolkit: Draw it, Connect it and Run it[END_REF][START_REF] Macintyre | DART: a Toolkit for Rapid Design Exploration of Augmented Reality Experiences[END_REF]. We identified another factor that distinguishes authoring of post-WIMP user interfaces: many post-WIMP interface elements (e.g. gestures, speech input) have no graphical representation in the user interface and therefore make it more difficult to graphically represent them in author-

The paper is organized as follows: First, related work is presented to illustrate the state of the art in authoring for post-WIMP applications. Then our cross layer authoring concept and CLAY (Cross Layer Authoring Tool) are being introduced and illustrated with a small use case. After explaining the basic navigation elements of the tool, we focus on the creation and configuration of post-WIMP functionality. Finally, we discuss our concept and the CLAY editor by evaluating user feedback to our prototype and stimulating possible future research topics.

Related Work

There exist several frameworks and libraries which enable programmers to add post-WIMP features to existing applications or to develop applications with post-WIMP user interfaces, e.g. PyMT [START_REF] Hansen | PyMT: a Post-WIMP Multi-Touch User Interface Toolkit[END_REF] for touch or the Kinect SDK 1 for full body interaction. Those frameworks provide powerful tools for programmers, but are less fitted for interface designers [START_REF] Kin | Proton: Multitouch Gestures as Regular Expressions[END_REF][START_REF] Klemmer | Papier-Mache: Toolkit Support for Tangible Input[END_REF]. In the following, we focus on concepts assisting programmers on the one hand and supporting visual development (for the GUI designer, post-WIMP designer or client) on the other hand. One approach to handle the complexity of the development of post-WIMP applications is to enhance a classical WIMP UI by merging post-WIMP capabilities on the same abstraction layer, as can be found in Apple's authoring tool named Xcode2 , for instance. With Xcode, the WIMP UI can be created graphically in a GUI editor. The UI elements are then bound to UI events by dragging graphical connection lines to methods or properties specifically marked in the source code. However, there is no visual support to add post-WIMP input (e.g. custom gestures) to the UI, so it can hardly be achieved without fundamental knowledge of the required programming techniques. This might lead to the programmer and designer having to spend more time negotiating specifications, which may add a significant overhead and prohibits focus on each author's individual task. Also, Xcode's concept does not include support for testing gestures directly while creating them. To inspect the created behavior, the use of a simulator or device is required. This makes the authoring process of post-WIMP capabilities inefficient, because the entire resulting application must be compiled and executed to test the behavior every time the logic or the parameters of the specific gesture change.

While Xcode's approach focuses on multi-touch interactions, there are other concepts of authoring tools that support the development of applications featuring all kinds of post-WIMP interactions. One of them is Squidy, a zoomable design environment for natural user interfaces [START_REF] König | Squidy: a Zoomable Design Environment for Natural User Interfaces[END_REF]. It employs other concepts like the MaggLite authoring toolkit for post-WIMP interfaces [START_REF] Huot | The MaggLite Post-WIMP Toolkit: Draw it, Connect it and Run it[END_REF] or the OpenInterface Framework [START_REF] Serrano | The Openinterface Framework: a Tool for Multimodal Interaction[END_REF], which offer a way to create post-WIMP user interfaces with the concept of visual dataflow programming. Squidy addresses the issue that the authors involved in the authoring process of post-WIMP UIs often use multiple toolkits or frameworks to create the desired UI and its behavior. This is achieved by tying together relevant frameworks and toolkits in a common library, while a visual language is introduced to create natural user interfaces. To see the current development status as a whole, Squidy introduces the concept of semantic zooming, making it possible to control the level of complexity shown to the particular author developing the application. However, Squidy has a focus on providing a way to take input data of hardware devices (e.g. movement data recorded by a Kinect), process the data and send it to listening applications, e.g. via TUIO [START_REF] Kaltenbrunner | reacTIVision and TUIO: a Tangible Tabletop Toolkit[END_REF]. This focus on technical aspects is important to develop post-WIMP input alternatives actually available for a specific application. Still Squidy lacks the possibility to design the user interface and to add the created post-WIMP input methods to it. In addition, the capabilities of adding and validating the post-WIMP input on the actual application UI are missing. Thus, designers without any technical knowledge about the specific implementation of the post-WIMP input may have difficulties using the technical base and design as well as validating the UI as a whole.

Hartmann et al. present a rapid prototyping tool called Exemplar [START_REF] Hartmann | Authoring Sensor-based Interactions by Demonstration with Direct Manipulation and Pattern Recognition[END_REF] that allows for authoring of sensor-based interactions by demonstration, thus enabling short iterative demonstrate-edit-review cycles. Yet, it only covers one step in the application development process, forcing the author to switch between different tools, whereas we seek to embed interactive editing (particularly of post-WIMP behavior) seamlessly into the overall authoring process.

Juxtapose [START_REF] Hartmann | Design as Exploration: Creating Interface Alternatives Through Parallel Authoring and Runtime Tuning[END_REF], a prototyping code editor and runtime environment for UIs, allows for creating multiple alternatives of application logic and real-time tuning of application parameters. Juxtapose facilitates the work of UI developers, yet it only addresses authors with both UI design and programming knowledge, as the parameters are directly extracted from the code. The CLAY concept shares the goal to enable rapid parameter testing, however, it strives to provide it on a more abstract level for UI designers who do not necessarily know how each parameter is implemented in the program code. Generally, the Juxtapose concept lacks the distinction between different author roles.

Simpson and Terry [START_REF] Simpson | Embedding Interface Sketches in Code[END_REF] point out the problem of the use of heterogeneous toolsets for application development and present GUIIO, a design tool which uses ASCII text for rendering UI mock-ups. While it offers a fluid transition from program code to UI, it does not offer possibilities to inspect the UI in the way it would actually present itself to the user, nor does it provide means to graphically represent post-WIMP interaction. Instead of just visualizing the connection between code and UI from the perspective of the coder, we introduce a concept comprising the points of view of all the authors involved in the process, allowing them to work together with only one tool.

DENIM [START_REF] Newman | DENIM: An Informal Web Site Design Tool Inspired by Observations of Practice[END_REF] is an informal website design tool presented by Newman et al. in 2003. It allows for informal sketch-based prototyping of websites and offers different representations of the sites through zooming. Similarly to CLAY, DENIM uses a slider with eleven levels to navigate from site map to storyboard and individual page view, thus enabling an almost smooth transition. Yet, the zooming in DENIM is only used to reveal more or less details of only one field of activity (the graphical user interface design), whereas CLAY uses seamless transitions to navigate between views of entirely different natures (user view, post-WIMP functionality, classic GUI and code). So instead of zooming "horizontally" on one aspect of an application, the CLAY concept allows for a "vertical" zoom through the different areas of expertise of the authors involved.

To sum up, authoring tools with a focus on a specific platform provide visual means to develop the WIMP part of the UI. Yet the development of post-WIMP capabilities has to be done entirely in the source code by programmers with specific technical knowledge. Besides, the resulting behavior is hard to inspect without compiling the application and testing on the target device [START_REF] Khandkar | Tool Support for Testing Complex Multi-Touch Gestures[END_REF]. More general approaches which support adding all kinds of post-WIMP capabilities to applications often have a focus on technical aspects. They lack the support for designing the application's UI, integrating post-WIMP capabilities and visually testing the added post-WIMP interactions. Different authoring roles are not differentiated enough to allow authors to concentrate on their specific contribution to the post-WIMP application.

Cross Layer Authoring

We distinguish between four different author roles in the process of a post-WIMP application development: the coder, the GUI designer, the post-WIMP designer and the user. We regard the authors' contributions to the application being developed as layers placed above one another, with the code layer being at the bottom. Above that, there are the GUI layer, the post-WIMP layer and the user layer. The code layer can be regarded as the fundament of the application, which all other layers depend upon. Likewise, the post-WIMP layer builds upon the interface layout and traditional elements (such as buttons, text fields) created in the GUI layer. Each layer is designed to suit the tasks and skills of the respective author only, e.g. source code can only be edited in the code layer, graphical interface design only takes place in the GUI layer. Apart from distinguishing between author roles, we further split up the task areas of each of these roles into four editor modes, namely content, layout, parameters and behavior, which indicate what kind of changes can currently be made to the application. Dividing all the editable attributes into four different modes aims at keeping the screen organized and minimizing the risk for accidental changes (e.g. changing the position of a text label when intending to change its content). An overview of the resulting sixteen development modes and the respective functionality available in each mode can be found in Figure 1.

During the authoring process, cross layer connections between GUI components, post-WIMP input methods and the application logic on the code layer have to be created. By visualizing the connections between the layers when navigating between them, information about how the layers work together can be provided to each author. Transitions between the author layers are designed to suit the authoring task at hand and to happen seamlessly in order to avoid disorientation due to abrupt changes in the UI of the authoring tool.

In order to be able to evaluate this concept, we implemented the CLAY editor. CLAY is realized as a classic desktop application as from our observation many developers prefer a PC-based environment with multiple screens. Development espe-cially on smartphones with small screens does not seem to be practical. Nevertheless, the concept could be adapted to work on specific post-WIMP devices as well.

Content

The CLAY Prototype

In the CLAY user interface, we place graphical representations of the previously described layers and editor modes at the borders of the authoring tool (see Figure 2). They are arranged as two orthogonal axes forming a two-dimensional matrix where one dimension serves to distinguish between the authoring layers and the other dimension is used to switch the editor mode. With this, we provide a simple mental model how the authoring functionality in the tool is structured. There is constant visual feedback in which state the authoring tool is set.

The slider on the left-hand side allows for a smooth transition between author role layers. When the slider is positioned directly over a layer icon, only the respective layer is visible at 100 percent opacity. When the slider is moved towards a different layer, the current layer becomes more and more translucent to show information from the layer the slider is being moved to. This blending should eliminate the need for entirely switching between layers in many cases. If, for instance, a button is connected to a code action, selecting the button on the GUI layer and moving the slider towards the code layer will result in the connected method shining through the GUI layer. When approaching the code layer, the semantic level of detail of the code information increases until the slider has reached the code layer icon, where the whole code file is shown in a text editor (see Figure 3). In contrast to a navigation where layers are discretely switched, we expect the slider to provide a better orientation and understanding of connections between layers. Furthermore, the interface is divided up into two frames: a main frame in the center and a smaller frame on the right. The main frame serves to type code or put together GUI or post-WIMP elements, depending on the active layer. The right frame on the other hand is used to display the elements or options available in the current layer and editor mode. On the uppermost layer, the user layer, there is no right frame. Instead, the main frame is made wider to make more room for the screens of the application being inspected.

On the horizontal axis at the top, the developer can switch between the four editor modes content, layout, parameters and behavior. When switching between editor modes, the right frame's content adapts accordingly to provide a list of the specific components or parameters needed.

The play button in the bottom left can be accessed from all layers at any time. Pressing the button results in the application being compiled and run in a simulator in full screen mode. With the target device connected, this button also allows for a test run on the specific device. Though the resulting application can always be inspected in the user layer, it is important to provide a simulator where no other graphical elements of the authoring tool itself are visible in order to get a good understanding for the resulting application. It is even more important to provide this functionality as CLAY runs on desktop computers where post-WIMP input can often only be approximated by WIMP input.

The CLAY Authoring Process

Throughout the next sections we illustrate the CLAY authoring process employing the following use case: Restaurant owner R has furnished his restaurant with multitouch tables and needs a tabletop software which should allow his guests to skim through the food menu, place orders and play games while they are waiting for their meals.

There are three authors involved in the development process: Coder C is responsible for the actual functionality of the program developed. GUI designer G is responsible for the selection and layout of classical WIMP user interface elements and the transitions between screens. Post-WIMP designer P works on the screens provided by the GUI designer and adds post-WIMP functionality to the application. Restaurant owner R might be involved in the process at times, and may customize the product to a restricted extent.

Our tool provides views for each of these four roles. The view of the end user, a guest in the restaurant, is included in form of a simulator. In the following section, we elaborate in the challenges of each of the authors, introduce their layers and the associated editor modes in detail and motivate the need for smooth transitions between layers.

Code Layer. The programmers need a text editor with syntax highlighting and refactoring options in order to implement the custom GUI and post-WIMP component classes and the methods called by them. In addition to writing code, they have to provide hooks for the upper layers to enable them to connect to their methods and use their classes without intruding into the source code.

In our concept, these hooks are called components and action elements, which are being created by the coder as abstract representations for classes and methods. They create a level of abstraction for the upper layers and allow for exchangeability on the code layer without the GUI and post-WIMP layers having to perform any changes. These abstract elements also allow for simultaneous development across the layers, as the actual implementation of e.g. a specific method does not need to be completed in order for the upper layers to be able to connect to it.

In our example, coder C creates a new file in the content mode. After implementing a method, e.g. for sending an order to the kitchen, C switches to the behavior mode where she can create and edit action elements which are displayed in the right frame. She has to provide a name and description for the action (e.g. "Send order to kitchen"), which will be displayed in the GUI and post-WIMP layers. Additionally, she can restrict the action to be only available to certain components like a button or a tap. In doing so, C would prevent G or P from inappropriate connections, e.g. the event created by interaction with a tangible object to a specific action. Wiring up the action with the code can be done graphically by dragging a line from the newly created action element in the right frame to the small circle next to the code. Circles are being displayed next to the starting point of classes and methods. An unfilled circle indicates that the method is not yet connected to an action element. After connecting, the circle displays a small green filled circle inside (see Figure 4). Creating GUI and post-WIMP components works similarly. In order to create e.g. a custom button, C switches to the layout mode where existing component elements are listed in the right frame. Along with giving a name and description, property access rights can be granted to the GUI and post-WIMP designer for every component and parameter individually in the parameters mode. For instance, the color and size of the button can be made editable, while the label can remain fixed. These access rights are not enforced in the tool, they serve rather as a reminder that an author did not deem it sensible to make certain alterations. Thus, by changing layers an author is able to override restrictions. GUI Layer. Graphical interface designers do not always have broad programming skills and may wish to focus on the design and layout of the screens. It is their job to put together standard components (screens, buttons, labels etc.) or specific custom components created in the code layer. Furthermore, certain code actions or screen transitions can be connected to them. As the GUI designers are usually not aware of the source code, they need an abstraction of the methods available to connect to.

We conceived the GUI layer to resemble already existing authoring tools where the creation of the application screens is being done graphically via drag and drop. In order to connect certain components to code methods, the action components come into play. They can be connected to the WIMP components via drag and drop as well, eliminating the need for the GUI designer to modify any of the source code.

In our scenario, graphical interface designer G creates a screen with two boxes and several buttons by dragging these components from the right frame to the canvas in the layout mode. The components can be rearranged and edited according to the active editor mode selected on the top slider: To change text labels or images, the author has to switch to content mode. Changes in size, shape or color can be performed in the parameters mode, according to the access rights granted to the components in the code layer. Actions and transitions between screens can be attached to components like buttons or table cells in the behavior mode. To connect the "Send order" method to a button, G drags a line from the associated action element on the right. When hovering over the elements on the canvas, those components potentially able to connect to an action are highlighted by rendering them in green color. Alternatively, a component can be selected by clicking first, then the right frame will only display the methods available for the specific component.

The transition from GUI layer to code layer is designed to allow for inspecting the connections from the user interface to the code. The amount of the information shown from the code layer increases as the user moves towards that layer. We illustrate this transition with our use case in the following. As soon as G selects the mentioned button, a small number appears on the code icon on the slider to indicate how many connections to the code the button has, in this case a "1" for the connected "Send order" method. Thus G can instantly see whether or not there is code layer information attached for further inspection. When moving the slider towards the code layer, a small pop-up appears next to the button showing the name of the source file containing the connected method. When moving further to the code layer, the pop-up displays the method name (see Figure 5), and in a next step the method source code. When the slider has reached the code layer, the complete source file containing the "Send order" method is being displayed.

Post-WIMP Layer. Post-WIMP designers complement the screens designed by the GUI designers with post-WIMP functionality like touch gestures, voice input or tan-gible object support. Such designers might be experts in usability, but maybe not know much about coding. We provide a visual authoring tool similar to the tools already available for graphically developing WIMP interfaces, where designers simply drag and drop post-WIMP elements, combine them or adjust their parameters. However, while classic WIMP UIs need to cope with visible input and output channels only (buttons, text fields etc.), post-WIMP interactions do not always have an instant graphic representation. While a button is visible and known to be clickable, the area for e.g. a swipe gesture does not need to have a perceivable boundary. Without a graphical representation, it is hard to detect which post-WIMP interaction is offered by an application.

Our goal was to create a post-WIMP layer that closely follows the concept of the GUI layer, i.e. it enables the author to graphically create input alternatives and connect them to code actions without the need to change the source code. In order to achieve this, we conceived possibilities to graphically assign post-WIMP functionality to application screen elements. The process of creating and inspecting post-WIMP elements will be further elaborated in the section "Authoring of Post-WIMP Interactions".

User Layer. Developers may want to take a look at the application screens they are currently working on without distracting connection symbols or tooltips and without having to compile the application first. Methods for user-centered design call for customers to be able to influence the development process from time to time without having to work with complex tools and risking to damage anything. That is why we included the user layer where screens are displayed the way they would look in the application when run on the target device. Here, editing is restricted to changes of the appearance or the content like exchanging texts or images, changing of colors and fonts, or rearranging of the components present. Thus, this layer may not only help developers during the authoring process, but also serves as a presentation mode for meetings with the clients.

In CLAY, the user layer consists of a big canvas showing the application's screens. When in behavior mode, the application can be inspected by moving the mouse over the screens and their elements. Whenever actions are connected to a component, they will be represented graphically. In our example, restaurant owner R may, for instance, adjust the colors to match his restaurant's interior in the parameters mode or change the texts of the food menu in the content mode. The goal is to include the customers in an iterative development process, enabling them to contribute their feedback more directly and allowing them to make changes to the application without the need to have to rely on the other authors.

Authoring of Post-WIMP Interactions

Standard authoring tools today are not designed for graphical authoring of post-WIMP interaction, but instead require programming knowledge to enhance their applications with post-WIMP features (see section 2). Our concept of cross layer authoring strives toward enabling the author of each layer to work independently, e.g. a post-WIMP designer does not necessarily need to perform changes on the code layer.

In this section, we illustrate our approach for graphically creating, combining and validating post-WIMP elements in this section by exemplarily describing some use cases of our previously introduced scenario.

Creating Post-WIMP Interactions. After the graphical UI of the application has been designed and developed, it is the task of post-WIMP designer P to enhance the interface with post-WIMP functionality. The application screen shows a box where a list of dishes is being displayed and a smaller box underneath. P wants to enable the user to scroll through the food menu by panning over the smaller box from left to right. In the layout mode, he drags a pan gesture onto the canvas resulting in a small translucent gray box containing the respective element symbol. As further configuration of the element is necessary, it is marked by a small yellow warning sign.

In order to connect a post-WIMP interaction to an application screen, the author has to define where the interaction can be conducted and which area, if any, is being affected by this action. For this, we conceived a visualization of the activity and output areas that can be defined for every post-WIMP interaction. Activity area in this context denotes the region where e.g. a touch gesture is being executed, while the optional output area denotes the region that responds to that gesture. After defining these areas, they are graphically highlighted to make it easier for the author to inspect the post-WIMP elements they created. Post-WIMP interactions do not necessarily affect elements of the user interface, they might also affect the application logic. In this case, a dedicated area is provided in the authoring tool that serves as a representation of the output area.

In our example, post-WIMP designer P wants the user to pan inside the box at the bottom, so P assigns the pan's activity area to that box by dragging from the pan symbol to the box while holding down the left mouse key. While dragging, a red arrow is being displayed pointing away from the post-WIMP element towards the cursor. After connecting, the bottom box shows a red overlay with a preview of the pan gesture, i.e. a small white circle, indicating a finger, moving up (see Figure 6). Fig. 6. Detail of the post-WIMP layer: A newly created pan gesture is being displayed in a small box. After connecting the pan to the area where it can be executed ("activity area"), the respective region shows a red overlay. A red frame around the pan symbol indicates that it has been connected to an activity area.

The pan gesture created by P provides default values containing the direction, the length and the duration of the pan, which were defined in the code layer. As coder C has made these values changeable, P can adjust them to his needs in the parameters mode, e.g. change the angle to make it a horizontal pan instead of a vertical one. The changes are instantly being reflected in the preview on the activity area.

Next, P defines the output area for the pan. In this example, he wants the box at the top to be scrolled whenever the user carries out a pan in the lower box. Assigning the output area works similarly to defining the activity area, except for now, P has to press the right mouse key and the displayed arrow and the accentuation of the components are painted blue. After connecting, the upper box has a blue overlay. A bicolored frame around the post-WIMP symbol indicates that it is connected to both an activity and an output area (see Figure 7). Connecting post-WIMP elements to code actions can be done in the same manner as in the GUI layer. When dragging the scrolling action to the newly created post-WIMP element, it is highlighted green. After successfully connecting the action, a small green filled circle is displayed in the upper right corner of the pan symbol (see Figure 7). As scrolling is a standard action in the tool, it also has a graphical representation, which is immediately visible on the output area overlay. Parameters like scrolling direction or speed can be set in the parameters mode along the lines of the configuration of the pan gesture.

Combined and Alternative Post-WIMP Elements. In addition to adding single post-WIMP elements to application screens, it is also possible to graphically combine several elements to one larger element group or to offer alternative input methods. This is a novel approach to deal with complex post-WIMP input, as it can be connected to a code action without the code layer having to know what exactly triggered the action.

In our next use case, P wants to add two alternative possibilities to order a meal on the food menu screen: the first is to perform a long press on the desired dish and say "Order" at the same time, the second is to perform a long press on the designated food menu item and then swipe to the right. To achieve this, P drags a long press gesture to the canvas, and then adds a voice input element by dropping it onto the long press box. Both elements are now arranged next to one another, indicating that they have to be performed consecutively. After defining the activity areas, configuring the elements in the parameters mode (e.g. setting the voice input to the word "Order") and wiring up the post-WIMP group with the corresponding code action, the setup of the first input alternative is completed.

To implement the second input method, P adds a swipe gesture to the group, which should be performed alternatively to the voice input. To achieve this, P drops the swipe element directly onto the voice input symbol, which results in the swipe being displayed above the voice input. This indicates that either both actions have to be performed at the same time or alternatively. The behavior for these kinds of stacked elements can be switched by clicking on the label appearing above the elements, reading either "Together" or "Choice" (see Figure 8). Further configuration concerning e.g. the time interval between the long press and the swipe or voice input can be configured in the parameters mode. No changes have to be made in the behavior mode as the post-WIMP group has already been connected to a code action that will be triggered whenever one of the alternative chains of post-WIMP interactions has been performed.

Visual Validation

In order to be able to inspect the post-WIMP interactions, which have been added to an application, we conceived what we call visual validation. It is meant to visualize the sequence of a specific post-WIMP interaction (or group) with their respective activities and output areas. This enables the author to test the course of the post-WIMP interactions without having to compile the application first, thus targeting on a reduction of the time needed for testing and adjusting the interaction parameters [START_REF] Khandkar | Tool Support for Testing Complex Multi-Touch Gestures[END_REF].

In our CLAY prototype, visual validation can be performed in the content mode. When switching to that mode, all post-WIMP boxes turn into little players with a timeline and a play button (see Figure 9). Pressing play starts a real-time animation of the post-WIMP elements on the screen they are connected to. Touch gestures are once more previewed by a graphical representation of the user's finger moving over the activity area, standard output elements are being represented by e.g. an arrow moving along the scroll direction. This is especially useful when several elements have been combined to one more complex element group, as their interplay can be inspected chronologically. In case a post-WIMP element directly triggers a code action, this is shown by an arrow directed towards an application logic icon, which then appears in the bottom right corner of the canvas. Having these players in the content mode aims at making it easy to inspect the temporal sequence of the post-WIMP elements and check the result at a glance. Additionally, the players can be organized in different groups, which should lead to more clarity on the screen. These groups can be added, shown and hidden in the right frame when in content mode.

User Feedback and Discussion

We evaluated our concept by presenting the prototype to three computer scientists and two media designer to gather qualitative feedback. All participants were between 20 and 40 years of age and had at least fundamental knowledge and experience regarding the design and development of post-WIMP applications.

The organization of the tool in four author layers was generally appreciated and found to be valuable, especially for large projects, though there was disagreement concerning the need for a user layer. While one subject found this layer particularly useful to include customers into the authoring process, another participant stated that customers might have no interest in actively participating in the development.

The division into four editor modes was regarded as useful. The fact that, for instance, the content mode offers rather different functionality on each layer was not regarded as confusing.

The concept of fading between layers using a slider instead of switching discretely was generally appreciated and regarded as enjoyable. Still, some participants pointed out the importance of the possibility to switch between layers faster, especially when using the tool for a longer time.

Creation of component and action elements in the code layer and usage of those elements in the upper layers was considered easy and intuitive. However, all subjects missed a possibility for the interface and post-WIMP designers to predefine custom components and actions they need the programmer to implement. In the CLAY concept, applications are built from the code layer to the upper layers, according to specifications negotiated between the authors. Future work might engage in a concept to allow the definition of custom components or actions not only in the code layer, but also in the GUI and post-WIMP layers.

It was generally considered problematic to create large applications with the tool as it was expected to become gradually overloaded. Nevertheless, our concept offers several approaches to handle complex applications. For instance, the author has the possibility to display connections between layers for every component individually and on demand. Post-WIMP element groups can be minimized to take up less screen space and organized hierarchically in groups, which might lead to a less overloaded screen. When inspecting several connections to the code layer at once, the author can at some point choose which of the boxes to expand in order to see the full source code of only the specific source file of interest.

The visual creation and validation of post-WIMP interactions was considered innovative and helpful, though some explanations had to be given concerning the setup of the interactions, alluding to the fact that it is not an intuitive concept which would benefit from explanatory tool tips.

In conclusion, all participants stated that they generally found the CLAY concept to be an improvement of the authoring process of post-WIMP applications and would consider working with a tool based on that concept in the future.

Conclusion

We presented a concept called cross layer authoring for a tool enabling the collaborative development of post-WIMP applications for several different author groups. Layers in this context denote the contribution of the particular author groups to the developed application. We identify four different layers, namely the code, GUI, post-WIMP and user layer, each corresponding to an author role. Furthermore, we split up the tasks for each of these author roles into the four editor modes content, layout, parameters and behavior. We suggest organizing cross layer authoring tools in form of a matrix, with one dimension serving to distinguish the author layers, and the other dimension used to switch between editor modes, which received consistently positive feedback in our user interviews. Our goal is to provide a tool enabling authors to work on their respective tasks independently. In order to allow UI designers to work autonomously, we include possibilities to graphically create the GUI and post-WIMP functionality. A concept for visually enhancing user interfaces with post-WIMP input methods has been introduced. Furthermore, the graphical creation and inspection of combined post-WIMP interactions has been presented and found to be innovative and useful by the users. In order to enable connections between layers, we introduce the concept of abstract connection hooks. These connections can be graphically inspected by traversing the layers. We propose a seamless transition between layers, which we realized in our prototype CLAY with a slider as a tool for navigation which users found easy and enjoyable to use. Future work might engage in a concept to soften the hierarchy of the layers, allowing GUI and post-WIMP designers to create custom component or action elements they need the programmer to implement. This might lead to a more bidirectional communication between layers and thus encourage the concurrency of the development process, which might result in a reduction of the development time.

Fig. 1 .

 1 Fig. 1. Overview of the different development modes of CLAY and their respective functionality: We distinguish between four author roles (leftmost column) and four editor modes (top row)

Fig. 2 .

 2 Fig. 2. The CLAY interface: A slider on the left allows for navigation through the author layers, a slider at the top can be used for switching editor modes.

Fig. 3 .

 3 Fig.3. Three phases of the smooth transition from the GUI to the code layer: As the slider on the left is moved from the GUI to the code layer, the interface builder becomes translucent and reveals more and more code layer details.

Fig. 4 .

 4 Fig. 4. Detail of the code layer: Code methods are tagged with a small circle. If connected to an action element, which makes them available to the GUI and post-WIMP layer, they also display a filled green circle within.

Fig. 5 .

 5 Fig.5. Two phases of the transition from GUI layer to code layer: A small pop-up first displays the source file only (left image), then also the method the selected GUI button is connected to (right image).

Fig. 7 .

 7 Fig. 7. Detail of the post-WIMP layer: The bicolored frame around the pan gesture symbol indicates that its activity and output areas are assigned. The green filled circle shows it is connected to a code method.

Fig. 8 .

 8 Fig.8. Detail of the post-WIMP layer: More complex post-WIMP interactions can be created by combining single gestures or events. In this case, a long press combined with either a swipe or voice input form a combined post-WIMP element, which can be connected to an action.

Fig. 9 .

 9 Fig. 9. On the post-WIMP layer, visual validation of e.g. a pan gesture can be performed in the content mode. Pressing the play button in the "Browse menu" box starts a real-time animation of the post-WIMP elements on the screen they are connected to.

Microsoft Kinect for Windows: http://www.microsoft.com/en-us/kinectforwindows/

Apple Developer: https://developer.apple.com/

Acknowledgements

This research work has been financially supported by the German Federal Ministry of Education and Research (BMBF-FHProfUnt grant no. 17043X10).