"The indexer should be intelligent, widely read, well acquainted with publishing practices, and capable of working rapidly to meet almost impossible deadlines.
Less time is available for the preparation of the index than for almost any other step in the bookmaking process…
In addition to requiring intense intellectual concentration, good indexing requires reflection; that is, the indexer should stop frequently, review what has been done, and decide whether both author and reader have been well served by the choices made."
~Chicago Manual of Style, 13th ed., 17.23-17.24