The measurement of elements in an assembly task-the information output of the human motor system

The performance of three well-practised subjects on an assembly task was analysed using both an electrical contact technique and high-speed film recording. The task consisted of transferring a row of pegs into a parallel row of holes 8 inches away. There were four variations in the diameter of the holes presenting different degrees of difficulty. An information analysis of the output of the motor response, based on a formula suggested by Fitts, was applied to the data. Differences both in the measuring techniques and in the way the information analysis was applied raised problems which have an important bearing on the thesis of the constancy of human channel capacity and on the possible use of information measures in analysing motor tasks. Discrepancies between the photographic and the contact analysis as techniques for defining Movement Loaded and Positioning elements were examined. Whilst the contact analysis showed movement time increasing with decreasing target tolerance the film analysis showed gross movement time to be constant and the variation due to difficulty confined to the relatively small secondary positioning movement. An attempt to analyse the information load on gross and secondary movement respectively indicated that gross movement was made to an area of 2 in. in diameter about the actual target irrespective of its size and that secondary movement began as this target was entered. Values for information transmission rate were found to be constant when based on the individual element times from the photographic analysis but different transmission rates were found when calculations were based on overall times. Finally some assumptions underlying the formula are examined and it is pointed out that other rational assumptions when applied to these data would lead to results which would not support the thesis of the constancy of human channel capacity.