The performance of information retrieval systems can be evaluated in a number of different ways. Much of the published evaluation work is based on measuring the retrieval performance of an average user query. Unfortunately, formal proofs are difficult to construct for the average case. In the present study, retrieval evaluation is based on optimizing the performance of a specific user query. The concept of query term accuracy is introduced as the probability of occurrence of a query term in the documents relevant to that query. By relating term accuracy to the frequency of occurrence of the term in the documents of a collection it is possible to give formal proofs of the effectiveness with respect to a given user query of a number of automatic indexing systems that have been used successfully in experimental situations. Among these are inverse document frequency weighting, thesaurus construction, and phrase generation.