A comparison of field and laboratory studies shows that the variability of visual estimates of airplane speed made by ground observers increases in direct proportion to the mean estimate in the same manner as does the variability of estimates made for quite different moving objects at close range in the laboratory. Since the standard deviation may be used as a measure of sensitivity to differences in speed, it follows that an observer discriminates a constant percentage difference throughout the range of aircraft speeds studied. This rule also seems to be applicable to certain aspects of automobile driving and to tracking of targets on a cathode-ray tube.
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