AbstractThe indexing of a document is among the most crucial steps in preparing that document for retrieval. The adequacy of the indexing determines the ability of the system to respond to patron requests. This paper discusses this process, and document retrieval in general, on the basis of formal decision theory. The basic theoretical approach taken is illustrated by means of a model of word occurrences in documents in the context of a model information system; both models are fully defined in this paper. Though the main purpose of this paper is to provide insights into a very complex process, formulae are developed that may prove to be of value for an automated operating system. The paper concludes with an interpretation of recall and precision curves as seen from the point of view of decision theory.