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This study investigates sequential dependencies in interresponse times {lRTs) produced by human subjects under random-ratio (RR) and staircase stochastic reinforcement of waiting (staircase SRW) schedules using autocorrelation and relative fmquencies of IRTn *IRTn+1 pairs. In the first of two experiments conducted for this study, subjects clicked a computer mouse either to gain or to lose pOints exchangeable for money according to probabilities specified under each of these schedules. The autocorrelation functions of IRT series showed sequential dependencies in IRTs. The relative frequencies of IRTn *IRTn+1 pairs following response-cost showed clear sequential dependencies, but those following reinforcement showed individual differences. The second experiment compared sequential dependencies in IRTs early in learning with those after responses had stabilized under the same schedules. Classifying the relative frequencies of IRTn *IRTn+1 pairs by consequence of response yielded detailed descriptions of individual response patterns.