Easy and difficulty Fitts's Law task performance is correlated


            Fitts’s Law states that  movement time, T, is related linearly to the logarithmic ratio of the distance to the target width.   All explanations for Fitts’s Law posit the potential for two phases of control, a distance-covering phase, potentially followed by a closed-loop error correction phase.    According to Crossman and Goodeve (QJEP, 1963;1983) adherence to Fitts’s Law is the result of the time needed for the closed-loop correction phase.  On the other hand, Meyer et al. (Psych. Review, 1988) posit that Fitts’s Law is the result of the choice of the duration of the initial submovement, and that the submovement duration choice adheres to the linear speed accuracy trade-off principles outlined by Schmidt et al. (Psych. Review, 1979).  We decided to investigate this matter in an indirect fashion by examining individual differences in Fitts’s Law performance as a function of difficulty.  As tasks become more difficult, the closed loop portion of the movement should become more important.  However, according to Meyer and colleagues, it is the speed-accuracy characteristics of the individual that determines Fitts’s Law behavior.  Twenty-three subjects performed six trials at each of eight Fitts’s Law repetitive aiming tasks.  Distance was either 12.5 or 25.0 cm and target width was 0.25, 0.50, 0.75 or 1.00 cm.  The ID values ranged from 4.64 to 7.64 bits.  We found that virtually all of the tasks were correlated for the Index of Performance (IP).  In other words, the correlation values were not scaled to the similarity of the ID values, as we inferred should be the case if visual feedback processing was more important at high ID values.  Thus, these results, we believe, are more consistent with the Meyer et al. (1988) optimized sub-movement model of Fitts’s Law.