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When ``documents'' were first introduced as part of the FOA process, it was as one of the set of potential, pre-defined answers to users' queries. Here we will ground this abstract view in practical terms that can be readily applied, for example to the searches that are now so common on the Web. Our goal will be to balance this practical description of how search engines work today with the abstract FOA view that goes beyond current practices to other kinds of search still to come.

A useful working definition is that a DOCUMENT is a {\em passage of free text}. It is composed of text, strings of characters from an alphabet. We'll typically make the (English) assumption that uses the Roman alphabet, Arabic numerals and standard punctuation; complications like font styles (italics, bold), and especially non-Roman MARKED ALPHABETS that add characters like \term{\"{a}}, \term{\c{C}}, \term{\~{N}}, \term{\ae}, etc., and the iconic characters of Asian languages, require even more thought.

By ``free'' text we mean it is in natural language, the sort native readers and writers use easily. Good examples of free text might be a newspaper article, a journal paper, a dictionary definition. Typically the text will be grammatically well-formed language, in part because this is {written} language, not oral. People are more careful when constructing written artifacts that last beyond the moment. Informal texts like email messages, on the other hand, help to point to ways that some texts can retain the spontaneity of oral communication, for better and worse [REF803] .

Finally, we will be interested in PASSAGES of such text, of arbitrary size. The newspaper example makes us imagine documents of a few thousand words, but journal articles make us think of samples ten times that large, and email messages make us think of something only a tenth as long. We can even think of an entire book as a single document. All such passages satisfy our basic definition - they might be appropriate answers to a search about some topic.

The length of the documents will prove to be a critical issue in FOA search engine design, especially when the corpus contains documents of widely varying lengths. The reason is, roughly, that since longer documents are capable of discussing more topics, they are capable of being about more. Longer documents are more likely to be associated with more keywords, and hence more likely to be retrieved (cf. Section §3.4.2 ).

One possible response is to make a simple but very consequential assumption:

All documents have equal \about-ness.

In other words, if we ask the ({a priori}) probability of any document in the corpus being considered relevant, we will assume all are equiprobable. This would lead us to {\em normalize} documents' indices in some way to compensate for differing lengths. The normalization procedure is a matter of considerable debate; we will return to consider it in depth later (cf. §3.4.2 ).

For now, we will take a different tack towards the issue of document length, as captured by an alternative pair of assumptions:

The smallest unit of text with appreciable \about-ness is the paragraph.

All manner of longer documents are constructed out of basic paragraph atoms.

The first piece of this argument is that the smallest sample of text that can reasonably be expected to satisfy a FOA request is a paragraph. The claim is that a word, even a sentence, does not by itself provide enough {context} for any question to be answered, or ``found out about.'' But if the paragraph has been well-constructed, as defined by conventional rules of composition, it should answer many such questions. And unless the text comes from James Joyce, Proust, or Lois Borges, we can expect paragraphs to occupy about half an average screen page -- nicely viewable chunks.

Assumption (FOAref) alludes to the range of structural relationships by which the atomic paragraphs can typically be strung together to form longer passages. First and foremost is simple sequential flow, the order in which an author expects the paragraphs to be written. The sequential nature of traditional printed media, from the first papyrus scrolls to modern books and periodicals, has meant that a sequential ordering over paragraphs has been dominant. It may even be that the modern human is especially capable of understanding {\em rhetoric} of this form (cf. §6.2.3 ).

In any case, a sequential ordering of paragraphs is just one possible way they might be related. Other common relationships include:

\item {\em hierarchical} structure composing paragraphs into sub-sections, sections, and chapters. \item {\em footnotes}, embellishing the primary theme; \item {\em bibliographic citations} to other, previous publications; \item references to other sections of the same document; especially \item {\em pedagogical prerequisite} relationships ensuring that conceptual foundations are established prior to subsequent discussion;

Of course each of these relationships has grown up within the tradition of printed publication. Special typographical conventions (boldface, italics, sub- and superscripting, margins, rules) have arisen to represent them and distinguish them from sequential flow.

But new, electronic media now available to readers (and becoming available to authors) need not follow the same strictly linear flow. The new capabilities and problems of traversing text in nonlinear ways -- HYPERTEXT -- have been discussed by some visionaries [REF701] [Nelson87] for decades. This new technology certainly permits us to make some traversals more easily (e.g., jumping to a cited reference with the click of a button rather than via a trip to the library), but this same ease may make it more difficult for an author to present a cogent argument.

For now we will not worry about just how arguments can be formed with nonlinear hypermedia. Assumptions(FOAref) and (FOAref) simply allow us to infer Assumption (FOAref) : If all the documents are paragraphs, we can expect them to have virtually uniform `aboutness'. These too are simplifying assumptions, however. In an important sense a scientific paper's abstract is about the same content as the rest of the paper, and a newspaper article's first paragraph attempts to summarize the details of the following story. These issues of a text's LEVEL OF TREATMENT will be discussed later.


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