As we just described, static filtering models work under the assumption that the
filtering profile remains static over time. In some ways, this makes the filtering
process easier, but in other ways it makes it less robust. All of the popular static
filtering models are derived from the standard retrieval models described in Chapter
7. However, unlike web search, filtering systems do not return a ranked list of
documents for each profile. Instead, when a new document enters the system, the
filtering system must decide whether or not it is relevant with respect to each profile.
Figure 10.6 illustrates how a static filtering system works. As new documents
arrive, they are compared to each profile. Arrows from a document to a profile
indicate that the document was deemed relevant to the profile and returned to
the user.
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