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We've introduced keywords as features mentioned by users as part of
their queries, but the other face of keywords is as descriptive features
of documents. That is, we might naturally say that a document is
about \term{ROBOTICS}. Users mentioning \term{ROBOTICS} in their
query should expect to get those documents that are about this
topic. Keywords must therefore also function as the {\em documents}'
description language. The same vocabulary of words used in queries must
be used to describe the topical content of each and every document.
Keywords become our characterization of what each document is \about.
INDEXING is the process of associating one or more keywords with
each document.
The vocabulary used can either be CONTROLLED or
UNCONTROLLED (a.k.a., CLOSED- or OPEN-VOCABULARIES
). Suppose we decide to have all the documents in our corpus manually
indexed by their authors; this is quite common in many conference
proceedings, for example. If we provide a list of potential keywords and
tell authors they must restrict their choices to terms on this list, we
are using a controlled indexing vocabulary. On the other hand, if we
allow the authors to assign any terms they choose, the resulting index
has an uncontrolled vocabulary [Svenonius86] .
To get a feel for the
indexing process, imagine that you are given a piece of text and must
come up with a set of keywords that describe what the document is Let's
make the exercise more concrete. You are the author of a report entitled
``Using a neural network for prediction,'' and are submitting it to a
journal. One of the things this particular journal requires is that the
author provide up to six keywords under which this article is going to
be indexed. If you are sending it to the {\em Communications of the
ACM}, you might pick a set of keywords that identify, to the audience of
computer scientists you think read this publication, connections between
this new work and others' prior work in related areas.
\term{NONLINEAR
REGRESSION; TIME SERIES PREDICTION} \smallskip
But now imagine instead
that you've decided to submit the {exact same paper} to {\em Byte
Magazine}, but must again pick keywords that have meaning to this
audience. You might choose:
\term{NEURAL NETWORKS; STOCK MARKET ANALYSIS}
\smallskip
What is the {context} in which these keywords are going to be
interpreted? Who's the audience? Who's going to understand what these
keywords \mean? Anticipating the FOA activity in which these keywords
will function, we know that the real issue to be solved is not only to
describe this one document, but to {\em distinguish} it from the
millions of others in the same corpus. How are the keywords chosen going
to be used to distinguish your document from others?
It is often easiest
to imagine keywords as independent features of each document. In fact,
however, keywords are best viewed as a { relation} between a document
and its prospective readers, sensitive to both characteristics of the
users' queries and other documents in the same corpus. In other words,
the keywords you pick for Byte should be different than those
you pick for {\em Communications of the ACM} and for deeper reasons than
what we might cynically consider ``spin control.''
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