8. User interfaces and usability

The person who uses a library is conventionally called a "patron" or "reader." In computing, the usual term is "user" or "end user". Whatever word is chosen, digital libraries are of little value unless they are easy to use effectively.

The relationship between people and computers is the subject of intensive research, drawing on field as diverse as cognitive science, graphic design, rhetoric, and mathematical modeling of computer systems. Some of the research aims to develop a theoretical understanding of how people interact with computers, so that models of how people process information can be used to design appropriate computer systems. Other research helps the user comprehend the principles behind a computer system, to stimulate productive use of the services and information that it provides. This chapter, however, is less ambitious. It concentrates on the methods that are widely used today, with some illustrations of experimental systems that show promise.

Change is one of the themes of digital libraries and change is one of the problems in designing user interfaces. Traditional libraries are not easy to use effectively, but they change slowly; users develop expertise over many years. Digital libraries evolve so quickly that every month brings new services, new collections, new interfaces, and new headaches. Users do not enjoy having to relearn basic skills, but change appears to be a necessary evil.

Partly because of this rapid change, users have an enormous variety in their levels of expertise. Much of the development of digital libraries has come out of universities, where there are many experts. Colleagues and librarians are at hand to help users, while system administrators configure computers, install software, and track changes in the market place. As the Internet has spread more widely, digital libraries are being used by people who do not have access to such expertise and do not want to spend their own time learning techniques that may be of transitory value. This creates a tension. Advanced features of a library are valuable for the specialist. They enable the skilled user to work faster and be more effective, but digital libraries must also be usable by people with minimal training.

Aspects of usability and user interface design

In discussing the usability of a computer system, it is easy to focus on the design of the interface between the user and the computer, but usability is a property of the total system. All the components must work together smoothly to create an effective and convenient digital library, for both the patrons, and for the librarians and systems administrators.

Figure 8.1. Aspects of usability

Figure 8.1 shows a way to think about usability and the design of user interfaces. In any computer system, the user interface is built on a conceptual model that describes the manner in which the system is used. Here are some typical conceptual models that are used to design digital libraries. In practice, most digital libraries combine concepts from several such models.

• The classical library model is to search a catalog or index, select objects from the results, and retrieve them from a repository.

• The basic web model is for the user to follow hyperlinks between files.

• The Z39.50 protocol supports a conceptual model of searching a collection, and storing sets of results for subsequent manipulation and retrieval.

The right-hand side of Figure 8.1 shows the layers that are needed to implement any conceptual model. At the top is the interface design, the appearance on the screen and the actual manipulation by the user. This covers such considerations as fonts, colors, logos, key board controls, menus, and buttons. The next level down, the functional design, represents the functions that are offered to the user. Typical functions include selection of parts of a digital object, searching a list or sorting the results, help information, and the manipulation of objects that have been rendered on a screen. These functions are made possible by the two bottom sections of the figure: the data and metadata that are provided by the digital library, and the underlying computer systems and networks. Panel 8.1 illustrates these five aspects by showing how they apply to an important application.

The desk-top metaphor

Almost all personal computers today have a user interface of the style made popular on Apple's Macintosh computers, and derived from earlier research at Xerox's Palo Alto Research Center. It uses a metaphor of files and folders on a desktop. Its characteristics include overlapping windows on the screen, menus, and a pointing device such as a mouse. Despite numerous attempts at improvements, this style of interface dominates the modern computer market. A user of an early Macintosh computer who was transported through fifteen years and presented with a computer running Microsoft's latest system would find a familiar user interface. Some new conventions have been introduced, the computer hardware is greatly improved, but the basic metaphor is the same.

In the terminology of Figure 8.1, the conceptual model is the desktop metaphor; files are thought of as documents that can be moved to the desktop, placed in folders, or stored on disks. Every windows-based user interface has this same conceptual model, but the interface designs vary. For example, Apple uses a mouse with one button, Microsoft uses two buttons, and Unix systems usually have three. The functions to support this model include open and closing files and folders, selecting them, moving them from one place to another, and so on. These functions differ little between manufacturers, but the systems differ in the metadata that they use to support the functions. The desktop metaphor requires that applications are associated with data files. Microsoft and Unix systems use file naming conventions; thus files with names that end ".pdf" are to be used with a PDF viewer. Apple stores such metadata in a separate data structure which is hidden from users. Finally, differences in the underlying computer systems permit some, but not all, user interfaces to carry out several tasks simultaneously.

Browsers

The introduction of browsers, notably Mosaic in 1993, provided a stimulus to the quality of user interfaces for networked applications. Although browsers were designed for the web, they are so flexible that they are used as the interface to almost ever type of application on the Internet, including digital libraries. Before the emergence of general purpose browsers, developers had to provide a separate user interface for every type of computer and each different computing environment. These interfaces had to be modified whenever the operating systems changed, a monumental task that few attempted and essentially nobody did well. By relying on web browsers for the actual interaction with the user, the designer of a digital library can now focus on how to organize the flow of information to the user, leaving complexities of hardware and operating systems to the browser.

Basic functions of web browsers

The basic function of a browser is to retrieve a remote file from a web server and render it on the user's computer.

• To locate the file on the web server, the browser needs a URL. The URL can come from various sources. It can be typed in by the user, be a link within an HTML page, or it can be stored as a bookmark.

• From the URL, the browser extracts the protocol. If it is HTTP, the browser then extracts from the URL the domain name of the computer on which the file is stored. The browser sends a single HTTP message, waits for the response, and closes the connection.

• If all goes well, the response consists of a file and a MIME type. To render the file on the user's computer, the browser examines the MIME type and invokes the appropriate routines. These routines may be built into the browser or may be an external program invoked by the browser.

Every web browser offers support for the HTTP protocol and routines to render pages in the HTML format. Since both HTTP and HTML are simple, a web browser need not be a complex program.

Extending web browsers beyond the web

Browsers were developed for the web; every browser supports the core functions of the web, including the basic protocols, and standard formats for text and images. However, browsers can be extended to provide other services while retaining the browser interface. This extensibility is a large part of the success of browsers, the web, and indeed of the whole Internet. Mosaic had three types of extension which have been followed by all subsequent browsers:

• Data types. With each data type, browsers associate routines to render files of that type. A few types have been built into all browsers, including plain text, HTML pages, and images in GIF format, but users can add additional types through mechanisms such as helper applications and plug-ins. A helper application is a separate program that is invoked by selected data types. The source file is passed to the helper as data. For example, browsers do not have built-in support for files in the PostScript format, but many users have a PostScript viewer on their computer which is used as a helper application. When a browser receives a file of type PostScript, it starts this viewing program and passes it the file to be displayed. A plug-in is similar to a helper application, except that it is not a separate program. It is used to render source files of non-standard formats, within an HTML file, as part of a single display.

• Protocols. HTTP is the central protocol of the web, but browsers also support other protocols. Some, such as Gopher and WAIS, were important historically because they allowed browsers to access older information services. Others, such as Netnews, electronic mail and FTP, remain important. A weakness of most browsers is that the list of protocols supported is fixed and does not allow for expansion. Thus there is no natural way to add Z39.50 or other protocols to browsers.

• Executing programs. An HTTP message sent from a browser can do more than retrieve a static file of information from a server. It can run a program on a server and return the results to the browser. The earliest method to achieve this was the common gateway interface (CGI), which provides a simple way for a browser to execute a program on a remote computer. The CGI programs are often called CGI scripts. CGI is the mechanism that most web search programs use to send queries from a browser to the search system. Publishers store their collections in databases and use CGI scripts to provide user access. Consider the following URL:

  http://www.dlib.org/cgi-bin/seek?author='Arms'

  An informal interpretation of this URL is, "On the computer with domain name "www.dlib.org", execute the program in the file "cgi-bin/seek", pass it the parameter string "author='Arms'", and return the output." The program might search a database for records having the word "Arms" in the author field.

  The earliest uses of CGI were to connect browsers to older databases and other information. By a strange twist, now that the web has become a mature system, the roles have been reversed. People who develop advanced digital libraries often use CGI as a method to link the old system (the web) to their newer systems.

Mobile code

Browsers rapidly became big business, with new features added continually. Some of these features are clearly good for the user, while others are marketing features. The improvements include performance enhancements, elaboration to HTML, built-in support for other formats (an early addition was JPEG images), and better ways to add new formats. Two changes are more than incremental improvements. The first is the steady addition of security features. The other is mobile code which permits servers to send computer programs to the client, to be executed by the browser on the user's computer.

Mobile code gives the designer of a web site the ability to create web pages that incorporate computer programs. Panel 8.2 describes one approach, using small programs, called applets, written in the Java programming language. An applet is a small program that can be copied from a web site to a client program and executed on the client. Because Java is a full programming language, it can be used for complex operations. An example might be an authentication form sent from the web site to the browser; the user can type in an ID and password, which a Java applet encrypts and sends securely back to the server.

Java is not the only way to provide mobile code. An alternative is for an HTML page to include a script of instructions, usually written in the language known as JavaScript. JavaScript is simpler to write than Java and executes quickly. A typical use of JavaScript is to check data that a user provides as input, when it is typed, without the delays of transmitting everything back to the server for validation. Do not be confused by the names Java and JavaScript. The two are completely different languages. The similarity of names is purely a marketing device. Java has received most of the publicity, but both have advantages and both are widely used.

Recent advances in the design of user interfaces

The design of user interfaces for digital libraries is part art and part science. Figure 8.1 provides a systematic framework for developing a design, but ultimately the success of an interface depends upon the designers' instincts and their understanding of users. Each part of the figure is the subject of research and new concepts are steadily being introduced to the repertoire. This section looks at some of the new ideas and topics of research. In addition, Chapter 12 describes recent research into structural metadata. This is a topic of great importance to user interfaces, since the manner in which digital objects are modeled and the structural metadata associated with them provide the raw material on which user interfaces act.

Conceptual models

Several research groups have been looking for conceptual models that help users navigate through the vast collections now available online. There are few landmarks on the Internet, few maps and signposts. Using hyperlinks, which are the heart of the web, the user is led to unexpected places and can easily get lost. Users of digital libraries often work by themselves, with little formal training, and nobody to turn to for help. This argues for interfaces to be based on conceptual models that guide users along well-established paths, although, with a little ingenuity, some people are remarkably adept at finding information on the Internet. Observations suggest that experienced people meet a high percentage of their library needs with networked information, but less experienced users often get lost and have difficult evaluating the information that they find. Panel 8.3 describes two research projects that have explored novel conceptual models for digital libraries.

Interface design

Interface design is partly an art, but a number of general principles have emerged from recent research. Consistency is important to users, in appearance, controls, and function. Users need feedback; they need to understand what the computer system is doing and why they see certain results. They should be able to interrupt or reverse actions. Error handling should be simple and easy to comprehend. Skilled users should be offered shortcuts, while beginners have simple, well-defined options. Above all the user should feel in control.

Control creates a continuing tension between the designers of digital libraries and the users, particularly control over graphic design and the appearance of materials. Many designers want the user to see materials exactly as they were designed. They want to control graphical quality, typography, window size, location of information within a window, and everything that is important in good design. Unfortunately for the designer, browsers are generic tools. The designer does not know which browser the user has, what type of computer, how fast the network, or whether the display is large or small. Users may wish to reconfigure their computer. They may prefer a large font, or a small window, they may turn off the display of images to reduce network delays. Therefore, good designs must be effective in a range of computing environments. The best designers have an knack of building interfaces that are convenient to use and attractive on a variety of computers, but some designers find difficulty in making the transition from traditional media, where they control everything, to digital libraries and the web. A common mistake is over-elaboration, so that an interface is almost unusable without a fast network and a high-performance computer.

One unnecessary problem in designing interfaces is that Netscape and Microsoft, the two leading vendors of browsers, deliver products that have deliberately chosen to be different from each other. A user interface that works beautifully on one may be a disaster on the other. The manufacturers' competitive instincts have been given priority over the convenience of the user. If a designer wishes to use some specialized features, the user must be warned that the application can not be used on all browsers.

Functional design

Digital libraries are distributed systems, with many computers working together as a team. Research into functional design provides designers with choices about what functions belong on which of the various computers and the relationships between them. Between a repository, where collections are stored, and the end user, who is typically using a web browser, lies an assortment of computer programs which are sometimes called middleware. The middleware programs act as intermediaries between the user and the repository. They interpret instructions from users and deliver them to repositories. They receive information from repositories, organize it, and deliver it to the user's computer. Often the middleware provides supporting services such as authentication or error checking, but its most important task is to match the services that a user requires with those provided by a repository.

Figures 8.2 and 8.3 show two common configurations. In both situations a user at a personal computer is using a standard web browser as the front end interface to digital library services, illustrated by a repository and a search system.

Figure 8.2. User interface with interface scripts

Figure 8.2 shows middleware implemented as independent computer programs, usually CGI scripts, that run on server computers somewhere on the network. The browser sends messages to the interface in a standard protocol, probably HTTP. The scripts can run on the remote service or they can communicate with the service using any convenient protocol. In the figure, the link between the interface scripts and the search system might use the Z39.50 protocol. The technical variations are important for the flexibility and performance of the system, but functionally they fill the same need. This configuration has the disadvantage that every action taken by the user must be transmitted across the network to be processed and the user then waits for a response.

Figure 8.3. User interface with mobile code (applets)

Figure 8.3 shows a more modern configuration, where mobile code is executed on the user's computer by the browser. The code might be Java applets. Until used, the code is stored on the remote server; it is loaded into the user's computer automatically, when required. This configuration has many advantages. Since the user interface code runs on the user's computer it can be very responsive. The protocol between the user interface and the services can be any standard protocol or can be tailored specially for the application.

A presentation profile is an interesting concept which has recently emerged. Managers of a digital library associate guidelines with stored information. The guidelines suggest how the objects might be presented to the user. For example, the profile might recommend two ways to render an object, offering a choice of a small file size or the full detail. The user interface is encouraged to follow the profile in rendering the data, but has the option of following different approaches. An potential use of presentation profiles is to permit specialized interfaces to support people who have physical difficulties in using the standard presentation.

Computer systems and networks

The performance of computer systems and networks has considerable impact on the usability. The creator of a digital library can make few assumptions about the equipment that a user possesses, beyond the basic knowledge that every user has a personal computer attached to the Internet. However, this simple statement covers a multitude of situations. Some personal computers are more powerful than others; the quality of displays vary greatly. Some people have their own private computer, which they can configure as they wish; others may share a computer, or use more than one computer. Any digital library must assume that users will have a variety of computers, with various operating systems and environments. The environments include the various versions of Microsoft's Windows, the Macintosh, and a plethora of types of Unix. Differences between operating systems can be minimized by using a web browser for the user interface, but performance differences are not so simple.

The usability of a computer system depends upon the speed with which it responds to instructions. The designer of a digital library has little or no knowledge of the quality of network connections between the user and the library. Connections vary from spectacularly fast to frustratingly slow and even the best connections experience occasional long pauses. A user interface that is a delight to use over a 10 million bits/second local network may be unusable over an erratic, dial-up link that rarely reaches its advertised speed of 14 thousand bits/sec. Thus digital libraries have to balance effective use of advanced services, requiring fast equipment and up-to-date software, with decent service to the users who are less well provided.

Improving the responsiveness of browsers

Web browsers incorporate several tricks to improve the responsiveness seen by the user. One is internal caching. Information that has been used once is likely to be used again, for example when the user clicks the "back" button on a web browser; some graphics may be repeatedly constantly, such as logos and buttons. Web browsers retain recently used files by storing them temporarily on the disk of the user's personal computer. These files may be HTML pages, images, or files of mobile code, perhaps in JavaScript. When the user requests a file, the browser first checks to see if it can read the file from the local cache, rather than reach out across the Internet to retrieve it from a distant server.

Another family of methods that improve the responsiveness of browsers is to carry out many operations in parallel. Browsers display the first part of a long file before the whole file has arrived; images are displayed in outline with the details filled in later as more data arrives. Several separate streams of data can be requested in parallel. In aggregate, these techniques do much to mitigate the slow and erratic performance that often plagues the Internet.

Mirroring and caching

A key technique to enhance the performance of systems on the Internet is to replicate data. If a digital library collection is to be used by people around the world, duplicate copies of the collection are made at several sites, perhaps two in Europe, two in the United States, one in Australia, and one in Japan. This is called mirroring. Each user selects the mirror site that provides the best performance locally. Mirroring also provides back-up. If one site breaks down, or a section of the network is giving trouble, a user can turn to another site.

While mirroring is a technique to make copies of entire collections, caching is used to replicate specific information. A cache is any store that retains recently used information, to avoid delays the next time that it is used. Caches are found within the computer hardware to help computer processors run quickly; they are found in the controllers that read data from computer disks; they are used to improve the speed and reliability of the domain name system which converts domain names to Internet addresses. Digital libraries have caches in many places. Organizations may run local caches of documents that have been read recently; a digital library that store large collections on slow but cheap mass storage devices will have a cache of information stored on faster storage devices.

All methods of replicating data suffer from the danger that differences between the versions may occur. When information has been changed, some users may be receiving material that is out of date. Elaborate procedures are needed to discard replicated information after a stated time, or to check the source to see if changes have taken place. Caches are also vulnerable to security break-ins. A computer system is only as secure as its weakest link. A cache can easily be the weakest link.

Reliability and user interfaces

Few computer systems are completely reliable and digital libraries depend upon many subsystems scattered across the Internet. When the number of independent components is considered, it is remarkable that anything ever works. Well-designed computer systems provide the user with feedback about the progress in carrying out tasks. A simple form of feedback is to have an animation that keeps moving while the user interface is waiting for a response. This at least tells the user that something is happening. More advanced feedback is provided by an indication of the fraction of the task that has been completed with an estimate of the time to completion. In all cases, the user needs a control that allows a time consuming operation to be canceled.

The term "graceful degradation" describes methods that identify when a task is taking a long time and attempt to provide partial satisfaction to the user. A simple technique is to allow users of web browsers to turn off the images and see just the text in web pages. Several methods of delivering images allow a crude image to be displayed as soon as part of the data has been delivered, with full resolution added later.

User interfaces for multimedia information

The types of material in digital libraries are steadily becoming more varied. User interface designs for collections of textual material may be inadequate when faced with collections of music, of maps, of computer software, of images, of statistical data, and even of video games. Each is different and requires different treatment, but some general principles apply.

• Summarization. When a user has many items to choose from, it is convenient to represent each by some small summary information that conveys the essence of the item. With a book, the summary is usually the title and author; for a picture, a thumbnail-sized image is usually sufficient. More esoteric information can be difficult to summarize. How does a digital library summarize a computer program, or a piece of music?

• Sorting and categorization. Libraries organize information by classification or by ordering, but many types of information lack easily understood ways of organization. How does the library sort musical tunes? Fields guides to birds are easy to use because they divide birds into categories, such as ducks, hawks, and finches. Guides to wild flowers are more awkward, because flowers lack the equivalent divisions.

• Presentations. Many types of information require specialized equipment for the best presentation to the user. High-quality digitized video requires specialized displays, powerful computers, high-speed network connections, and appropriate software. A digital library should make full use of these capabilities, if they exist, but also needs to have an alternative way to present information to users with lesser equipment. This inevitably requires the digital library to offer a selection of presentations.

To design digital libraries for different types of information, it is abundantly clear that there is no single solution. User interfaces have to be tailored for the different classes of material and probably also for different categories of user. Panel 8.4 describes one good example of a user interface, the Informedia digital library of digitized segments of video.

User interfaces and the effectiveness of digital libraries

During the past few years, the quality of user interface design has improved dramatically. It is now assumed that new users can begin productive work without any training. Most importantly, there are now numerous examples of fine interfaces on the Internet that others can use as models and inspiration. Standards of graphical design gets better every year. For D-Lib Magazine, we redesigned our materials three times in as many years. Each was considered elegant in its day, but needed a face lift a year later.

Good support for users is more than a cosmetic flourish. Elegant design, appropriate functionality, and responsive systems make a measurable difference to the effectiveness of digital libraries. When a system is hard to use, the users may fail to find important results, may mis-interpret what they do find, or may give up in disgust believing that the system is unable to help them. A digital library is only as good as the interface it provides to its users.