Fitts (1954) proposed an index of difficulty (ID) metric which reflects the information generated in executing a movement of specified amplitude to a of specified width. The formula developed by analogy to the information theory of continuous signals states that ID = log, 2A/W, where A is movement amplitude and W is width. The validity of this metric rests on several well known assumptions about movement performance. In particular, the spatial distribution of movement endpoints is assumed to be normal, and centered about the middle of the target of the movement. Welford (1968, p. 147) has pointed out that the information content of a normally distributed signal is approximately 4.1333 a (i.e., ay'iire), where a is the standard deviation of the distribution. Therefore, if width is equal to 4.133 G, then width accurately reflects the information in the distribution of movement endpoints, and ID will be properly calculated. In this case, about 96% of the movements should hit the target, since ±2.067 a (i.e., 4.133 = 2 X 2.067) includes 96% of the normal distribution. Thus, if more (or less) than about 4% errors are made, nominal width is an inaccurate reflection of the information in the movement endpoint distribution and a correction is necessary to derive an width equal to ±2.067 G for use in the ID formula above. For example, if more than 4% misses are made, effective width would be wider than the nominal width intended by the experimenter. Furthermore, a person may consistently overshoot (or undershoot) the intended target, thus producing an effective movement amplitude longer (or shorter) than the amplitude intended by the experimenter. Thus, an additional correction must be applied before computing 10. Typically, corrections are made by consulting tables of the normal distribution function and determining a z score associated with the probabilities of overshooting and undershooting the target. From these, both effective