Indexes make some research faster and some research seemingly slower.
In research, we generally only cite the index to a record when:
- the record is no longer extant and the index is all there is
- the index contains information about the indexed person that is not in the original record
Even if there is an index (which may be created by the original record creator; an individual knowledgeable with local families, sources, research methods, and indexing; or someone with questionable skills) that index may not contain the name we need. We still need to search the actual record page by page–just in case. I’d seen that with census records numerous times–where the name was there but the index created later was “off” in some way. I’ve even seen the handwritten index created by the clerk leave out an entry that I had to find manually.
The index is meant to lead you to a record. It is not the means by itself. Indexes created by the actual clerks in the locations where the records were created tend to be better, but there are always issues (as the 1851 marriage from St. Louis demonstrates). And sometimes those locally created indexes contain annotations that can be helpful.
And, if the index contains something not in the actual record, then it should be cited. The accuracy of that annotation is another matter entirely.