Macintosh requirements: 4 MB RAM, System 7.0 or later, hard disk with at least 2 MB free, CD-ROM drive, Apple Macintosh-compatible printer, 256-color monitor for viewing color plates. IBM requirements: IBM 386 or later, MS-DOS 5.0 or higher, VGA or Super VGA adapter for viewing color plates, 4 MB RAM, Windows 3.1 running in enhanced mode, hard disk with at least 2 MB free, CD-ROM drive, Hewlett-Packard or compatible printer.

Should internists use the new Harrison's Plus CD-ROM? As with any information resource, clinicians will use it if the benefits of doing so outweigh the costs. When evaluating the usefulness of this and other electronic information resources, three main factors should be considered: information content, functionality, and usability.

A clinical computing tool is no better than its information content, or knowledge base. Is the knowledge base comprehensive? Authoritative? Current? Evidence-based? Relevant? Familiar? Some computer tools for clinicians use well-established knowledge bases that have stood the test of time, but others depend on newly developed, specialized knowledge bases that have yet to be tested in clinical use. Harrison's CD-ROM and Harrison's Plus CD-ROM are based on a well-respected information source familiar to most internists, the 13th edition of Harrison's Principles of Internal Medicine.

To improve on the printed version of a medical knowledge source, an electronic version must provide functionality that is not possible in print. The compact size of CD-ROM products alone is not enough; this benefit is largely negated by the need for specialized hardware to access the information. Electronic information sources have two real advantages: rapid access and retrieval of information and integration of multiple types of information. The digital storage format of the CD-ROM allows comprehensive indexing, rapid searching, and quick retrieval. Hypertext linking allows instant navigation to related material located elsewhere. The digital format also allows for the integration of information that would otherwise be stored and accessed separately. The integration of multiple text sources, such as medical textbooks, allows users to search many texts at once. The integration of disparate types of sources, such as a medical textbook and a pharmacopoeia or a database of bibliographic citations, is also useful. The integration of text with graphics, audio, video, and animation allows a much richer, more complete presentation of information. The integration of medical knowledge sources with an electronic medical record can provide relevant medical knowledge for clinical practice at the point of care. An electronic product is better than a printed information resource largely to the extent that it capitalizes on these potential functional advantages.

Usability is the third essential element to consider when evaluating electronic information resources. Regardless of the quality of the information or the added functionality of an electronic source, clinicians cannot benefit if the software is poorly designed, its features hidden, and its commands so arcane that extensive training and practice are needed. Is the software simple to install? Does it conform to interface standards for the operating system with which it is used? Can its basic functions be easily learned by a novice with minimal training? Does the documentation give ready access to information about advanced functions? In short, does the product require clinicians to learn about computers, or does it enable them to learn more about medicine?

Using this framework, I evaluated Harrison's CD-ROM and Harrison's Plus CD-ROM on both the Windows and Macintosh platforms. After installation, I used the product to answer clinical questions that arose during my practice, to prepare handouts for house officers, and to solve a case presented to me for a CPC-like teaching exercise.

On both the Windows and Macintosh platforms, installation was a snap. The program conforms well to the Windows and Macintosh interface conventions so that it behaves as a user would expect. Basic operations were easy to learn; simple searches could be done with a minimum of initial learning effort. The software handles multiple word forms and synonyms without requiring a complex syntax, improving search effectiveness while requiring little from the user. The program comes with a brief printed manual; thorough, context-sensitive, on-line help; and a tutorial that explains and demonstrates both basic and advanced features.

The benefit of electronic search, retrieval, and navigation is immediately apparent. After the user enters a term in the search window, the software rapidly returns a list of all the occurrences of that term in the text. Using the Harrison's Plus CD-ROM, both Harrison's Textbook of Internal Medicine and the USP Drug Information for Health Professionals can be searched simultaneously. This is much like looking a term up in an index and then turning to each page on which the term appears, but in this case, the list is more complete, the "pages" are sorted in order of relevance, and the user simply clicks the mouse to skip between sections of the book. Similarly, figures, tables, and chapters referred to in the text can be viewed with a few clicks. Terms in the text can be selected and automatically entered in the search window, allowing the user to search for other occurrences of these terms.

I found this rapidity especially useful in solving a case presented to me for a CPC-like conference. Although I also used printed sources, being able to search the entire book and then rapidly move within the text as I explored diagnostic possibilities enabled me to progress more rapidly, making notes along the way and leaving electronic bookmarks where I needed them. Used in this way, an electronic textbook such as Harrison's CD-ROM is a useful tool for differential diagnosis.

The program enables the user to personalize the textbook. Electronic "sticky notes" permit annotations to be added where one wishes and retrieved at a later date. Electronic bookmarks allow ready access to frequently needed information. The user can customize the display of information according to personal preferences. Sections of text and figures and tables can be printed or saved to disk for incorporation into other documents.

I found these features especially useful in responding to clinical questions that arose during teaching rounds. I was able to rapidly search for the subject of interest; select relevant text, figures, and tables; and incorporate these into a handout for the ward team. I also used this material as the basis for transparencies for presentations. In a teaching setting, electronic sticky notes and bookmarks can guide others to relevant portions of the text in answer to specific clinical questions or as reading material for a course.

There are drawbacks both to this product and to electronic information sources in general. In some cases, too many mouse clicks are required to get to the desired information. For example, clicking the reference to some figures opens the caption to the figure rather than the figure itself, and another click is required to view the actual figure. Also, in some instances, only small portions of text are displayed on the screen so that numerous clicks are needed to review the rest of the relevant chapter. Although Harrison's Plus CD-ROM integrates Harrison's Textbook of Internal Medicine with the USP Drug Information for Health Professionals and incorporates figures from Harrison's textbook, it does not offer the type of multimedia integration that computer software for general audiences (such as encyclopedias) currently provide. Future versions may take better advantage of this opportunity. A chapter on mitral stenosis, for example, could contain not only the text describing the condition but also audio recordings of the auscultatory findings, video clips from the echocardiogram, and angiography or animation of angioplasty done to correct the condition.

In summary, Harrison's CD-ROM and Harrison's Plus CD-ROM are easily used products that take advantage of their electronic format to provide functional benefits. Harrison's CD-ROM in particular is a good value; it costs about as much as the print version of the textbook. I am keeping my print version of Harrison's Principles of Internal Medicine: It is more efficient and comfortable for reading in my favorite chair by the hearth. But in the teaching clinic and on the wards, where I need to rapidly find or integrate information, I find myself using electronic sources, including Harrison's CD-ROM, more and more often.