Two widely used criteria for evaluating the effectiveness of information retrieval systems are, respectively, the recall and the precision. Since the determination of these measures is dependent on a distinction between documents which are relevant on the one hand, and documents which are not relevant on the other to a given query set, it has sometimes been claimed that an accurate, generally valid evaluation cannot be based on recall and precision. A study was made to determine the effect of variations in relevance assessments on the average recall and precision values used to measure retrieval effectiveness. Using a collection of 1200 documents in information science for test purposes, it is found that large scale differences in the relevance assessments do not produce significant variations in average recall and precision. It thus appears that properly computed recall and precision data may represent effectiveness indicators which are generally valid for many distinct user classes.