A double-blinded experiment was carried out to investigate the effect of the stimulant caffeine on performance on a simple physical task. Thirty male college students were trained in finger tapping. They were then divided at random into three groups of 10 and the groups received different doses of caffeine (0, 100 and 200 mg). Two hours after treatment, each man was required to do finger tapping and the number of taps per minute was recorded. Does caffeine affect performance on this task? If it does, can you describe the effect?

Source: N.R. Draper and H. Smith (1981). Applied regression analysis (2nd edition), Wiley, New York