Came across these 12-year old stats recently:
Bad news I think…
1. If two groups of people construct thesauri in a particular subject area, the overlap of index terms will only be 60%.
2. Two indexers using the same thesaurus on the same document use common index terms in only 30% of cases.
3. The output from two experienced database searchers has only 40% overlap.
4. Experts’ judgements of relevance concur in only 60% of cases.
[Source: JAA Sillince, 1992, Literature searching with unclear objectives: a new approach using argumentation. On-line Review, 16 (6), 391-409]
I think that just goes to show that the quality and knowledge of your indexers (human or otherwise) is incredibly important.