Having multiple indexes complicates the maintenance of collection-wide
statistics. For example, it affects the spelling correction algorithm in Section
3.3 (page 56) that selects the corrected alternative with the most hits.
With multiple indexes and an invalidation bit vector, the correct number of
hits for a term is no longer a simple lookup. In fact, all aspects of an IR
system – index maintenance, query processing, distribution, and so on – are
more complex in logarithmic merging.