The Anomalous Behaviour of Precision in the Swets Model, and Its Resolution

M. H. Heine has shown that if one follows the retrieval procedure associated with Swets' model of an information retrieval system it is possible that the inverse relationship between Recall and Precision may not hold. In this paper we extend Heine's result to the case where the criterion‐parameter can assume discrete as well as continuous values. A plausible model of this kind is described and it is shown that for that model, Recall and Precision are in fact inversely related. The condition under which this relation may possibly not hold is then examined, and the conclusion is reached that this behaviour is an effect of the customary retrieval procedure, rather than anything intrinsic in the Swets model itself. An alternative logic is proposed in which the expected relationship is restored, as well as performance improved.