Are motor response elements additive?

Abstract

Even after appropriate sampling procedures have been used to determine average times required for various motions, critics argue against the procedure of adding these times together to obtain the expected time for a complete sequence of motions. Ghiselli and Brown (1955) suggested that removing a motion from a sequence of several motions did not necessarily reduce the time of the shortened sequence. Consistent with this point of view, Wehrkamp and Smith (1952) showed that the time of a travel motion depended on the type of manipulative movement before and after the travel. And Hall (1956) clearly demonstrated the interactions between motions, which would further argue against simple additivity of movements. For example, he found that for a job of fastening plastic labels to the tops of keys of adding machines, the mean time for grasping correlated significantly with the mean time for transporting rnaterial. Correlations between mocions ranging down to -.93 were reported. Despite this, Buffa and Lyman (1958) timed a sequence of 16 motions; eliminated six, calculated the average time for the six, and predicted quite accurately how much time the remaining 10 in the cycle would require. Who is right? Is there an interaction berween motions? Can the times for motions be cumulated to yield a simple total? The answers to both questions seem to be affirmative, and it will now be shown that no contradiction is involved if it is accepted that there are interactions between motions, yet simple additivity can still yield the total time required for a sequence of such interacting mocions. Moreover, the conditions under which both statements are true can be specified if we apply some deductions of Gulliksen (1950) about combining test scores, supported by empirical analysis by Guilford, Lowell, and Williams ( 1942 ) . Gulliksen concluded that, if a large number of scores are to be combined, or if the scores have high intercorrelations, it makes relatively little difference what positive weights are assigned. Only where a few scores are combined (3 to 10) and the average correlation is low (.5 or less), will weightings yield significantly more accurate totals than simple summations of the scores. Moreover, the weights have to vary considerably from each other to make much of a difference. On the basis of a review of the empirical findings, Guilford (1954, p. 447) concluded that differential weighting . . . usually pays little dividends when there are more than 10 to 20 items [in a test]. Assuming that time scores are not different from other kinds of scores, Gulliksen's and Guilford's conclusions provide a basis for understanding the apparent contradiction between Wehrkamp and Smith's results and those of Buffa and Lyman.