Final answer:
The question's details pertain to a study about the effect of caffeine on finger tapping rates, not on grading essays. There is no mention of 400mg of caffeine or essay grading performances in the information provided.
Step-by-step explanation:
The question provided does not contain information about a study on caffeine tolerance that involves essay grading with 400mg of caffeine. Instead, it references a study regarding the effects of caffeine on the rate of finger tapping among college men.
According to the information given, the study was designed to determine if caffeine affects the rate at which people can tap their fingers. In this double-blind experiment, subjects were divided into three groups and administered different amounts of caffeine: 0 mg, 100 mg, or 200 mg. Following the administration of caffeine, their finger tapping rates were measured. The findings of this study would help in understanding how different doses of caffeine could potentially influence motor functions such as finger tapping.