A model for movement time on data-entry keyboards

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Abstract
A model is presented for the effects of keyboard geometrical design on the movement time between keys, when the movements are carried out under visual control with single finger operation (such as on a data-entry keypad). It is shown that, for a given key centre spacing, the optimum key width occurs when the gap between adjacent keys is equal to the width of the finger pad (that part of the finger which is used for key actuation). Three experiments are reported related to keyboard design. In an experiment on. simulated keyboards and using five different probes (four metal probes of different widths and real fingers), it is found that results similar to the theoretical predictions are obtained in that a minimum in movement time occurs when the inter-key gap is about equal to the probe width. The effect is somewhat less with real fingers when compared with metal probes. In the second experiment, five real keyboards were tested with constant key spacing and with variable key size. A number of different amplitudes were used in this test to cover the range used in practice; thus movements were made both ballistically and under visual control in the different conditions. It was found that the major features of the model were supported in this experiment, although there were differences both within and between subjects. The optimum key size, at which both movement time and error rate were minimized, was about 6 to 8 mm (this is applicable only to the standard key spacing of 19 mm used in the experiment). In this experiment, the effect sizes were generally smaller than those found with metal probes, possibly due to the variation of finger size in the subjects or to adaptability of the human to the different experimental conditions. In the third experiment, reciprocal movements on a calculator keyboard showed good agreement with Pitts' Law when the effective target width, as defined in the paper, was used for calculation of the Index of Difficulty.

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