  There have been a number of prior attempts to theoretically justify the
effectiveness of the inverse document frequency (IDF). Those that take as their
starting point Robertson and Sparck Jones's probabilistic model are based on
strong or complex assumptions. We show that a more intuitively plausible
assumption suffices. Moreover, the new assumption, while conceptually very
simple, provides a solution to an estimation problem that had been deemed
intractable by Robertson and Walker (1997).
