There have been a number of major evaluations of the performance of retrieval systems against large full text and surrogate (bibliographic) databases. These evaluations have concentrated on the experimental determination of the Precision Ratio, the fraction of retrieved items that are relevant to an information request, and the Recall Ratio, the fraction of the total number of relevant items that were actually retrieved. While these measures have met with general acceptance, they have also generated much controversy. The purpose of this article is to review the results of several of the largest evaluations and to propose a simple model for the performance of such systems that may help explain the relationship between these measures and user behavior.