Alphabetical indexes

Most books have alphabetical indexes because, while we may know exactly what we are looking for, we often do not know where to find it. Though the alphabet is an arbitrary sequence of symbols, indexes work well because we have been taught alphabetical sequence from an early age. It is not universally useful, as you will find if you ever try to use a phone directory in a language that uses a different alphabet than your own. Many indexes are organized alphabetically, but few products are (primarily dictionaries, encyclopedias, and other reference works). This is because few data have any great meaning based on the first letter of their labels. The alphabet actually is a continuum (from A to Z) but it is a special one for the reasons stated above.

Site indices are detailed, alphabetical lists of terms that have more entries and less levels of hierarchy than tables of contents. Indices, like search engines, help users who know exactly what they're looking for. And, like tables of contents, they work because users have seen them before.

"According to Nancy Mulvaney (Indexing Books. Chicago: The University of Chicago Press, 1994) an index is "a structured sequence -- resulting from a thorough and complete analysis of text -- of synthesized access points to all the information contained in the text." What are the important points about this definition? First, that the index is a sequence, that is, it has a known order of items. While most indexes are arranged alphabetically, other orders are possible, such as numerical (for a parts list index) or chronological (for a timeline). But the index isn't just a list of entries, it is structured. In other words, the index shows relationships between various subjects, thus leading users to more specific or related topics that might meet their information needs more closely. Most importantly for the construction of an index, a human has looked at and analyzed the text. Although great strides have been made with the technology, automatic classification tools come nowhere near the human brain in terms of accuracy in evaluating text. There is simply too much contextual meaning that texts carry, too much social and cultural knowledge that while not stated in the text, needs to be accounted for when creating the index. Certainly no computer can yet understand the actual meaning of all texts. ...An index should lead users to relevant material, to significant content chunks that provide useful information, rather than to passing mentions of words." Fred Leise, 'Improving Usability with a Website Index

Indexes, as flat lists of terms, are easily scannable. Users need only use their browser's scroll bar to navigate through the entire index. (Large indexes often provide alphabetical anchor links at the top of the index, which take users quickly to the portion of the index they need to use.) There are no multiple levels to navigate, nor must users decide which branch of a hierarchy to click on, which often results in their missing information they are looking for or taking longer to find it. In fact, the easy scannability of the index on a single page is an important argument against having separate pages for letter of the alphabet, whenever possible. Through the use of multiple access points or "see" references, indexes help translate the vocabulary of the users to that of a text. In this example, for instance: cancer. See oncology The index is telling the user that this site does have information on cancer, but that it uses the term "oncology" to represent this concept. "See also" references assist users by helping them think about the information they are looking for."

A site index acts as an important complement to the site map or table of contents. Where the latter look at the high-level (or top-down) organization of information on the site, indexes look at the bottom-up view, that is, at specific, granular information chunks."
Leise, Fred, 'Improving Usability with a Website Index

Rosenfeld's Rules

see also: Rosenfeld, Lou, 'Organizing Your Site from A-Z'