We claimed (on page 80) that an auxiliary index can impair the quality of collection
statistics. An example is the term weighting method idf, which is defined as
log(N/dfi) where N is the total number of documents and dfi is the number of documents
that term i occurs in (Section 6.2.1, page 117). Show that even a small auxiliary
index can cause significant error in idf when it is computed on the main index only.
Consider a rare term that suddenly occurs frequently (e.g., Flossie as in Tropical Storm
Flossie).