A new test of throughput invariance in Fitts' law: role of the intercept and of Jensen's inequality

Fitts' law states that movement time varies linearly with the index of difficulty or, equivalently, that throughput (TP) is conserved across variations of the speed/accuracy strategy. Replicating a recent study by MacKenzie and Isokoski (2008), we tested the throughput invariance hypothesis with some fresh data and found the TP to be systematically affected by the strategy. This result, we suggest, pleads against the currently popular definition of the TP inherited from Fitts (1954), namely TP = ID/MT, which we recall is incompatible with the Shannon equation of Fitts' law. We also show that the statistical elaboration of the TP suffers from a problematic amount of uncontrolled variability due to the multiple inadvertent impact of Jensen's inequality.

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