A cost-benefit approach is employed to model the discretionary use of a computerized information system. The model is constructed by integrating a systemic view of human-computer interaction with a contingency model for selecting decision strategies. It is then used to analyse the effect of presentation format on search behaviour An experiment on matchmaking was performed using two formats: sequential and parallel. Subjects were asked to find the best spouse for a candidate using an information retrieval system that displayed information about the individuals according to the subject's specific requests. The subject's search behaviour and perception of complexity were recorded. A first analysis of the protocols revealed different behaviours, as predicted by the cost-benefit mechanism. A second analysis incorporated a measure of individual differences in perception to gain a better understanding of the effects of display format on perceived complexity and, thereby, on behaviour. The findings support the use or the cost-benefit model. The design implications of the model are also discussed.