User requirements and usage life -cycle for digital libraries

Margherita Antona1, Alexandros Mourouzis1, George Kartakis 1, Constantine Stephanidis1,2

1

1 Foundation for Research and Technology - Hellas (FORTH) Institute of Computer Science

GR-70013 Heraklion, Crete, GREECE cs@ics.forth.gr

2 University of Crete Department of Computer Science, GREECE

Abstract ?n the context of designing and developing a Digital Library, a significant issue to be taken under consideration is the user experience life-cycle. The identification and analysis of functional and non-functional requirements related to different life-cycle phases provide designers with appropriate clues for improving the usability and ultimately the acceptability of a Digital Library under design. This paper presents a generic usage life-cycle model for Digital Libraries, as well as a preliminary validation of the model based on the results of an empirical survey-based analysis of user requirements. The derived conclusions are that three main lifecycle phases (usage patterns) can be preliminary distinguished based on the correlation between user demographics such as usage experience and frequency and duration of use, and variations of users’ interest in functional and non-functional requirements. These three phases can be characterised as exploration in width, occasional (but long term) use and exploration in depth of a Digital Library user interface and content. The practical impact of the derived conclusions on the design of user interfaces of Digital Libraries is briefly discussed leading to indications for future work needed. 1 Introduction The design of a modern digital library (DL) is a complex task that usually requires the participation of a number of different parties, such as end-users, librarians, publishers, developers , as well as the formation of a multi-disciplinary team of design and human factors experts. A major challenge involved in such a process is to acquire and formulate the functional and non functional requirements that will drive appropriately the design of the DL in question. Identifying the characteristics and needs of target users and carefully studying relevant user tasks and requirements constitute a prevalent approach in this direction. The standard practice of digital libraries’ developers in order to elicit and cater for user requirements is to decide what information the library will contain, how that information will be generated, what audience the information is intended for, and how the data will be accessed (Richvalsky and Watkins, 1998). A more recent approach prompts designers and developers to adhere to a number of established principles for ensuring that the requirements of digital library stakeholders will be effectively meet. The design principles include the involvement of the appropriate experts during the development of the digital library, the thorough study of the digital material that will be integrated, the adoption of usability principles and standards related to digital libraries, as well as awareness of content persistence (McCray and Gallagher, 2001). The pertinent process to elicit user requirements involves conducting a survey in order to obtain an analytical description of potential user profiles, to create and study a list of the user’s current tasks related to the system’s process, to specify usability goals , to set the constrains and capabilities of the system under design, and to define the appropriate guidelines and standards (Mayhew, 2003). A dimension that appears to have a significant impact on user requirements is the long-term usage of DLs. As emerging repositories of digital resources and knowledge, DLs are likely to be visited repeatedly for long periods of time, ranging from casual to systematic usage. This has great impact, not only in terms of deployment and maintenance requirements, but also on the usage patterns, which in turn may have substantial impact on the user interface design requirements for the DL. Therefore, it is necessary to investigate user functional and non-

functional requirements in the light of the usage lifecycle in order to collect and consolidate knowledge reflecting the differentiating (design) issues related to the potential or desired evolution of the user. A line of work in this direction is currently being conducted in the context of the “User Interfaces and Visualization” work package of the DELOS Network of Excellence (see Acknowledgements). This paper reports on the preliminary specification of a generic usage life-cycle model for DLs , and its verification and further elaboration by means of an analysis of general functional and non-functional requirements for DLs gathered through an empirical survey conducted in the context of DELOS. 2 Functional and non-functional requirements One of the main objectives of the aforementioned work package of DELOS1 has been to collect and analyze general user requirements regarding the design of DLs, with the overall aim to provide a flexible design framework for DLs. A survey-based approach was employed in the context of an empirical investigation. Two separate questionnaires were elaborated and made publicly available through the Web2: one for digital library end-users and one for digital library stakeholders, such as content providers, librarians, etc. This paper focuses on the end-user requirements. The questionnaire addressed to DL end-users mainly consists of four parts, namely “User background and Demographics”, “Current Experience”, “Functional Requirements” and “Non-Functional Requirements ”. The full analysis of the results is available in Catarci (Ed., 2004). An overview follows of the gathered responses on these issues. 2.1 Overview of the user sample In total, 45 five responses (14 female, 25 male, 6 not specified) were received from the DL users online survey. The majority of the respondents range from 20 to 55 years, and all come from European countries. Almost half of the users are between 26 to 35 years old, while the second most frequent age is between 36-45 years, i.e., these ages are the most active in professional terms. The third age group is between 19 and 25, mainly students. The users’ scientific background is prevalently academic. A bias towards computer science was observed, but the samples contain very divergent backgrounds (from computer scientists to humanities studies and librarians). Concerning users’ professions, research activities are prevalent, along with engineering (either software engineering, or electrical and biomedical). Information services, which include librarians, information brokers etc., and professionals engaged in teaching activities , are also represented. In terms of disabilities, only few of the participants reported a considerable degree of disability in one or more of the following categories: cognitive, intellectual and visual. The sample of these 45 DL users is also characterised by multilingualism, high level of education (most of the participants have reached university education levels), high experience with computer systems and the Internet, as well as relatively high experience with DLs. Concerning the frequency of use of DLs, “twice a week” and “daily” are the ones with the highest rates. On the contrary, “seldom” and “monthly” frequencies obtained relatively low rates. This indicates that the users frequently access a digital library and thus they are aware of the weakness, advantages and drawbacks of the current digital systems. In addition, as far as the type of access used for data retrieval, the vast majority concerns public or free access, meaning that the users are not willing to pay for retrieving data and knowledge from a digital archive. Web access is by far the most popular (around 71%), ascertaining that Internet is more probable the most common medium for data retrieval. Reportedly, two thirds of the DLs used by the questionnaire respondents support English as the only language of interaction. Slightly less than a quarter of the DLs support both English and some other local language. Very few DLs offer multilingual support or only the local language. 2.2 Overview of functional user requirements In filling the questionnaire, users were asked to rate the importance to them of a number of common DL functions, including functions and facilities for: (i) locating resources; (ii) presenting resources; (iii) personalisation of content and services; (iv) communicating with other DL end-users; (v) collaboration with other DL end-users; (vi) a variety of other common DL services. Access to knowledge concerns functions for locating information, such as search, (social) navigation and browsing. Among these higher importance ratings were allocated to “search” (e.g., keywords search, parametric

1 http://delos.dis.uniroma1.it/default.aspx 2 http://www.dis.uniroma1.it/~delos/questionnaires

search), while moderate importance ratings to “index facilities” and “navigation-related functions” (e.g., browsing predefined catalogues), and lower importance ratings to “see also” items (e.g., items similar to the one at hand) and to “functions for filtering search/browsing results” (e.g., according to user profiles). Among the functions for personalisation of content and services, higher importance ratings were allocated to “functions for the presentation of contents according to profile”, and to “bookmarks facilities ”, moderate importance ratings to the “provision of suggestions for contents based on profile” and to “services offered for profile definition” (e.g., professional interests, personal interests, etc.), and lower importance ratings to the provision of “suggestions for discussion with other library members with similar interest profiles”, and to history facilities. Services for information access are rated by the users as follows: higher importance ratings for short description/previews, author(s)/editor(s), and title; moderate importance ratings for “popularity” (e.g., number of visits), “insertion /modification date” and “related items ”; and finally, lower importance ratings for “users’ scores” and “users’ discussions and reviews”. Among the functions for communicating and collaborating with other DL users, higher importance ratings were allocated to “shared annotation facilities” (e.g., peer reviews) and to “e-mail services ”, moderate importance ratings to “message boards services”, and lower importance ratings to “video conferencing and chat”. Among the rest of common DL functions, higher importance ratings were allocated to “printing and print preview facilities ” and to “downloading and uploading facilities ”, moderate importance ratings to “personal annotation”, and “notification (alerting) services”, and lower importance ratings to “social navigation support” (e.g., through users’ rating of content) and “multilingual support”. 2.3 Overview of non-functional user requirements In the questionnaire, DL users were also asked to rate the importance of a number of common usability and accessibility related requirements, as well as of other common non-functional requirements such as ethics, safety , privacy , security , and system performance. Among usability related requirements, higher importance ratings were allocated to “support for error prevention and handling”, and to “ease of use of the DL”, moderate importance ratings to “memorability”, and to “learnability’; and lower importance ratings to “subjective satisfaction for all levels of usage expertise (expert, moderate and novel). Reportedly, the “ease of use of a DL” is the most important feature. On the other hand, “satisfactory performance for the experts” is the lowest one. This means that users give high importance to the simple way of communicating, accessing, retrieving and searching for content of their interest. Furthermore, in terms of usability, some participants raised the importance of “speed and flexib ility in reaching information”, and mentioned that the digital library should ‘remember’ their usability and accessibility preferences and requirements. Among accessibility related requirements, higher importance ratings were allocated to support “for users with dexterity” and “for mobility impairments”, moderate importance ratings to support “for users with speech”, “hearing” and “learning impairments ”, and lower importance ratings to support “for users with reading” and “visual impairments ”. Among the rest of common non-functional requirements, higher importance ratings were allocated to “system performance”, moderate importance ratings to “security”, “privacy” and “safety”, and lower importance ratings to “ethical requirements”. 3 Digital Library usage life-cycle A preliminary investigation towards identifying a life-cycle model with regards to user experience with DLs has been conducted, with the purpose of analysing user requirements according to each life-cycle phase. The process followed towards this end included: (a) the elaboration of a generic life-cycle model; (b) the analysis of the data gathered through the questionnaire-based study in order to identify potential correlation between usage phases and user requirements ; and (c) the refinement of the life-cycle model based on the analysis of the empirical data. The subsequent subsections describe each of the steps above in more details.

3.1 Generic user experience life-cycle model

A preliminary normative model of user experience lifecycle has been elaborated, which is depicted in Figure 1. The proposed model is anticipated to be generic, and thus can apply not only to Digital Libraries, but to interactive technologies in general. The model suggests that when users are presented with a new software product or service, a number of expectations influence their decision about how and when they will use it, and notably perceived usefulness, i.e., the degree to which a person believes that using a particular system would enhance his or her job performance, and perceived ease-of-use, i.e ., the degree to which a person believes that using a particular system would be worth the effort. In these terms the proposed model complies with the Technology Acceptance Model (TAM, Davis, 1989, Davis et. al, 1989), which represents the factors determining technology acceptance by users. In Figure 1, these factors are reflected under the labels “Consider to Use” and “Consider to Reuse”. Besides these aspects, in the context of interactive products and services of the Information Society, a dimension which needs to be taken into account concerning user acceptance is that of accessibility, where accessibility is intended in the broader connotation of appropriate match between the user abilities, the characteristics of the context of use and of the access platform on the one hand, and the interaction facilities offered by the user interface on the other hand (Savidis et al. 2004)). This is reflected in Figure 1 under the label “Attempt to access”. Access ibility is a critical point of the entire process, which may resurface as a potential barrier at any stage, as the factors that may determine accessibility breakdowns are dynamic in the long-run (e.g., the user abilities may decrease over time). In the remaining of this paper, accessibility is considered as a prerequisite of usability.

Figure 1. Model of Digital library’s usage life-cycle (end-user)

3.2 Analysis of empirical data In order to obtain empirical evidence for further refining the model in a Digital Library specific perspective, and obtaining a view of the user functional and non-functional requirements related to each phase, the data gathered through the questionnaire have been analysed through cross-tabulation, with the objective of: (a) identifying different usage patterns (or usage levels), which constitute potential life -cycle phases; (b) validating the above through a correlation of user requirements to the identified potential lifecycle phases. Towards this end, a careful analysis of the demographics data of the 45 respondents to the questionnaire has been carried out and in particular data concerning users’ computing expertise, web usage frequency, digital library usage frequency and digital library usage sessions’ duration. Based on this analysis, three usage levels were identified. Level 1 includes users with comparatively less expertise, frequency and duration of computer and DL use, while Level 3 includes users with comparatively higher scores in the above. Level 2 users are statistically in the middle. The subsequent step of the performed investigation has been to analyse the data related to user functional and functional requirements gathered through the questionnaire according to the three identified levels, in order to

obtain an indication of the requirements associated to each level and detect significant patterns of variations , suggestive of different life-cycle phases characteristics and related users interactive behaviours . 3.2.1 Functional requirements vs. usage levels The data obtained through cross-tabulation of the identified usage levels and user functional related requirements are presented in Figure 2. Concerning the “location of information”, the interest of Level 3 users remain relatively higher (although with varying degrees) than the interest of the other two user categories in all considered functions, with the exception of “searching”, which appears to be considered as slightly more important by users of Level 1 (mean 8,75, where 1 represents minimum importance and 9 represents maximum importance), but also of Level 2 (8,60) with respect to Level 3 (8, 40). On the contrary, the interest of these user categories decreases for more advanced features such as “results filtering” (6 for Level 1and 6,75 for Level 2 vs. 8,25 for Level 3) and “see also facilities” (6,38 for Level 1 and 6,44 fo r Level 2 vs. 7,50 for Level 3). “Navigation related functionality” is also considered as less important by users of Level 1 (7,50) and Level 2 (7,09) with respect to users of Level 3 ( 8,25), while “indexing facilities ” are those with the nearest scores for all user categories (7,40 for Level 1, 7,30 for Level 2 and 7,75 for Level 3). “Resource presentation facilities” also involve significant differences among the three user groups, which however appear to converge on the high importance attributed to traditional presentation facilities such as “presentation by title” (8,73 for Level 1, 8,67 for Level 2 and 8,67 for Level 3) and “presentation by author/editor” (8,64 for Level 1, 8,67 for Level 2 and 8,33 for Level 3). Divergence emerges concerning the importance attributed to “short descriptions / previews” (6,91 for Level 1, 8,44 for Level 2 and 8,67 for Level 3), “related items” (5,56 for Level 1, 6,80 for Level 2 and 8,00 for Level 3), “users’ discussions and reviews ” (4,50 for Level 1, 4,25 for Level 2 and 6,67 for Level 1), and content item “popularity” (5,00 for Level 1, 5,00 for Level 2 and 6,00 for Level 2). The greatest divergence is observed concerning “users’ rating” (4,56 for Level 1, 3,86 for Level 2 and 7,33 for Level 3). Communication facilities show a variation from maximum to minimum interest for users of Level 1 and Level 3, and more stable scores for users of Level 2. “Email facilities” and “videos” are at the opposite extremes (7,57 for Level 1, 4,63 for Level 2 and 7,33 for Level 3, and 3,40 for Level 1, 4,38 for Level 2 and 3,33 for Level 3 respectively). Divergences occur for “shared annotation facilities” (5,71 for Level 1, 7,00 for Level 2 and 4,63 for Level 3) and “message boards” (5,40 for Level 1, 5,25 for for Level 2 and 4,33 for Level 3). Personalisation functions obtain significantly higher and more constant scores for users of Level 3 than for the other two categories. “Profile definition” obtains a high score for Level 3 (7,67), and significantly lower scores for Levels 1 and 2 (5,50 and 5,86 respectively). “Suggestions for contents based on profile” obtain a larger variation (5,71 for Level 1, 5,00 for Level 2 and 7,67 for Level 3). “Bookmarking facilities ” appear to be of relatively high interest to all categories (6,38 for Level 1, 6,88 for Level 2 and 7,67 for Level 3) along with “history facilities ” (5,13 for Level 1, 6,00 for Level 2 and Level 3). “Presentation of content” according to profile has similar degrees of interest for both Level 1 (6,75) and Level 3 (7,00) and lower degree for Level 2 (5,38). Finally, “suggestions for discussion” also presents significant score variation (4,75 for Level 1, 3,17 for Level 2 and 7,00 for Level 3). Other types of miscellaneous support have also been analysed. “Multilingual support ” shows similar high scores for the three levels (8,33 for Level 1 and Level 3, and 8,25 for Level 2). “Printing and Printing preview facilities” have higher scores for Level 1 and Level 2 users (7,90 and 7,88 respectively) and a slightly lower score for Level 3 users (7,33). “Glossaries” show a higher interest for Level 3 users (7,00) and lower interest for Levels 1 and 2 (5,00 and 5,29 respectively). “Personal annotation” and “notification / alerting services” have a high degree of interest for Level 3 users (7,00), a lower degree for Level 1 users (5,88) and an even lower degree for Level 2 users (4,86). An analogous pattern is also exhibited by “social navigation support”, with 4,71 for Level 1, 4,29 for Level 2, and 6,00 for level 3. Considering the above data, it can be observed that the three identified user levels exhibit different patterns of interest concerning DL functionality. Overall, users of Level 1 appear to be more interested in more traditional forms of functionality, such as search, navigation, presentation by title, author and insertion or modification date, indexing facilities, printing and print preview. Lower interest is expressed for communication functionality in general and for advanced functions such as notification and alerting, user rating, user profiling and discussion. On the other hand, users of Level 3 share interest in some traditional functions such as searching and navigation, but express a much higher degree of interest in more advanced features such as, for example, filtering, related

items, personalisation functions in general, and shared annotation. Users of level 2 express in many cases interests similar to those of Level 1, while in other cases they appear more kin to the use of more advanced features.

Figure 2. Functional requirements (mean values) vs. usage levels (sorted for Level 1 )

3.2.2 Non-functional requirements The data obtained through cross-tabulation of the three identified usage levels with interest in non functional requirements are summarised in Error! Reference source not found. (general requirements are represented on the left hand side and specific usability requirements appear on the right hand side). These data reveal a clear pattern of user concern regarding non functional requirements. Specifically, general non functional requirements are

much more significant for users of Le vel 2 than for users of Level 3, with users of Level 1 expressing intermediate degrees of interest. “System performance” obtains a score of 8,42 for Level 1, 8,78 for Level 2 and 6,80 for Level 3. Privacy obtains 6,58 for users of Level 1, 7,56 for Level 2 and 5,20 for Level 3, while “security” has similar scores (6,75 for Level 1, 7,89 for Level 2 and 5,00 for Level 3). “Safety” presents a relatively high score for Level 2 (7,89) and low score for Level 3 (4,40), while Level 1 obtains a middle score (6,42). Finally “ethical requirements ” have reduced variation among the three levels and comparatively lower scores (6,33 for Level 1, 6,67 for Level 2 and 5,00 for Level 3). The higher level of interest that the users of Level 2 have in this type of requirements with respect to users of Level 1 could be explained by the higher degree of awareness of the involved issues which is accumulated with usage experience, while the overall lower scores of Level 3 users may be due to increased acquaintance with the current DLs characteristics, and consequently decreases concern, which emerge during intensive or long-run usage. Concerning usability requirements, these appear to be in general more relevant for users of Level 1, followed by users of Level 2 and users of Level 3. The requirements related to user satisfaction show interesting scores as follows: expert user satisfaction obtains 5,83 for Level 1, 3,50 for Level 2 and 3,33, moderate user satisfaction obtains 4,83 for Level 1, 3,11 for Level 2 and 3,00 for Level 3, novel user satisfaction obtains 4,58 for Level 1, 4,00 for Level 2 and 3,25 for Level 3. These results may be interpreted as reflecting both the acquired experience of users in the three levels and their attitudes towards future usage, as for example in the case of Level 1 users, whose interest in expert user satisfaction show a corresponding interest towards future and more advanced usage pattern s. For other usability requirements, users of all levels appear to be more focused on their current needs while interacting with DLs. Ease of remembering obtains scores of 4,50 for Level 1, 3,22 for Level 2 and 2,50 for Level 3, while ease of learning obtains 4,50 for Level 1, 2,67 for Level 2 and 2,25 for Level 3, and ease of use obtains 4,08 for Level 1, 2,78 for Level 2 and 1,00 for Level 3. Finally, error prevention and handling has nearer scores of 3,83, 3,67 and 2,00.

Figure 3. Non functional requirements (mean values) vs. usage levels

3.2.3 Profiles of usage levels (distinct patterns of use) Based on the results of the empirical data, it can be concluded that the three identified usage levels are characterised by different degrees of interest in various DLs requirements, which can be analysed from the point of view of their interactive behaviour. Users of Level 1 have interests that reflect a wide non specialised exploration of DL interactive features and content, accompanied by a higher interest in more traditional and better known DL functions, a perceived need for common usability requirements such as learnability, memorability and ease of use, and a moderate level of attention for other commo n non-functional requirements. On the other hand, users of Level 3 are characterised by a pattern of specialised usage and in depth exploration, focusing on advanced interactive and communication features and recent or new content, with low concern for non-functional requirements in general, and only some moderate interest in productivity related aspects such as system performance. Level 2 appears to be characterised by a non explorative usage pattern, stabilised at the level of commonly used and known functions, with more limited interest in advanced features, and, on the other hand, a higher concern for non-functional qualities such as security and safety, and a moderate concern for some usability aspects, notably error prevention and handling. 3.3 Refine ment of the life-cycle model for Digital Libraries

The three usage levels identified and the profiles elaborated based on the empirical data suggest that three corresponding life-cycle phases can be identified, and namely: namely “Exploration in width” (Level 1), which refers to the phase of the preliminary familiarization of the user with the interactive environment of the Digital Library, “Occasional (long term) use” (Level 2), which refers to the phase of general use of a Digital Library, and “Exploration in depth” (Level 3), which refers to the specialized usage. The correspondence of usage levels with life-cycle phases is based on the rationale that the performed analysis of the empirical data validates the initial assumption that demographics characteristics of the user population related to previous usage experiences, frequency and duration are correlated to different usage behaviour patterns in DLS and with different types of functional and non-functional requirements.

Figure 4. The Evolved Digital Library Usage Lifecycle

Figure 4 illustrates how the generic usage lifecycle model described in section 3.1 has been refined reflecting the three phases discussed above. After accessing the system for the first time a (novice) user acquires (repeated) usage experience which can lead to (a) maintaining for some time an exploration in width usage pattern, or (b) adopting a more systematic / focused usage of the DL, thus entering any of the subsequent phases occasional use and in depth exploration or (c) abandoning the idea of using the system,. Nonetheless, phase changing

appears to take place as a gradual process, and this remains to be further investigated in more detail in future research.

4 Impact of the usage life-cycle model on the design of DL user interfaces From the conducted analysis and the elaborated life -cycle model, several preliminary design issues arise for DL user interfaces. A first consideration is that given the observed differences in usage patterns and interactive behaviours, but also the gradual nature of shifting from one phase to any of the subsequent ones , it is highly unlikely that a single user interface instance can satisfy the requirements of users in all the three phases, therefore leading to the need for user interfaces that are capable of evolving according to the user experience life-cycle phase of individual users. This requirement may be addressed through user adaptation and personalisation methods and techniques (e.g., Stephanidis et al, 2000), avoiding thereby the need of developing separate user interfaces for different phases, and allowing the possibility of personalised phase transitions. Furthermore, the results of this study can drive the des ign of three user interface instances, one for each of the main identified life-cycle phases. A designer, by taking under consideration the priorities set for each group regarding the potential functions and non-functional issues (e.g., these set forward from the results of this study), may produce three different designs that fully meet the corresponding requirements. In this direction, items of higher priority shall be more visible (easy to locate) and easy to use than those with lower priorities . Items of lower priority can afford larger number of interaction steps required in order to be reached and could be offered in a “more advanced options” form. These three user interface instances should be offered as options to any user at any time (e.g., through a user profile for user interface configuration, which should not be confused with the user’s interests profile which is actually associated with content access and not with interface design), and should be able to fully operate independently (i.e., the user should be able to access all possible functions from any of them - obviously in different ways). Once the three user interface instances have been fully defined and designed, then the designer, starting from the instance for the first usage phase, needs to introduce ideas for guiding the user, and ensuring graceful transformation, to the instance for the second or third phase depending on the goals of the user. In other words, the user interface needs to be able to evolve from one ‘version’ to another g radually based on the user feedback and confirmation for each change in question. A number of automations could be implemented in this direction for identifying the goals of the user at any time based on recorded behaviour and patterns of use. Finally, the user should be allowed at any time to select any of these user interface instances to freely and fully configure it in order to save it for personal use. In summary, the designers tasks / decisions in the context of the approach outlined above concern:

• Deciding upon all the functions that are to be supported by the system. • Based on the priorities set for the three main life-cycle phases, specifying separately three different

user interface instances (for items that are not part of the performed analysis, o ne could base design decisions upon the general model of the usage lifecycle and the identified general goals for each user level/type).

• Refining the designed user interface instances in order to support (automatic or semi-automatic) graceful transformation / transaction from one to another.

• Designing and integrate to the DL user interface a tool that allows full configuration and personalisation of any of the three user interface instances at any time.

5 Conclusions and future work This paper has proposed a design framework for DLs based on a usage experience lifecycle model that conforms with the generic technology acceptance lifecycle model (TAM) and which builds on empirical data emerging from the results of aDL user requirements survey. The results of the survey have been analysed in such a way as to capture correlation between participant demographics (overall computer use experience, Digital Library experience, frequency and duration of use, etc), and users functional and non-functional requirements. The correlation has allowed identifying three usage levels / types, namely (i) exploration in width, (ii) occasional (long term) use, and (iii) in depth exploration . These three levels, based on the corresponding user functional and non functional requirements, can be assumed to correspond to three different phases of the Digital Library user experience life -cycle , and drive appropriately the design of DLs that conform with the evolution of the users familiarity and expectation from the system. Future work will be targeted towards further validating the framework in question through: (a) a new survey that, based on the three identified phases, will directly ask users about

their needs and requirements concerning each phases; and (b) the development and user-based assessment of a pilot DL user interface that will follow the assumptions and guidelines emerging from this study. Acknowledgements

The work reported in this paper has been carried out in the context of the European Commission funded “DELOS Network of Excellence on Digital Libraries ” – DELOS NoE (IST-2002-23112). The authors would also like to acknowledge Evangelos Boutsakis for his significant contribution for the statistical analysis of the DELOS study results.

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