An experiment was conducted to compare the effects of four different drugs (A, B, C, and D) in delaying atrophy of denervated muscles. A certain leg muscle in each of 48 rats was deprived of its nerve supply by surgical severing of the appropriate nerves. The rats were then put randomly into four groups, and each group was assigned treatment with one of the drugs. After 12 days, four rats from each group were sacrificed, and the weight W (in grams) of the denervated muscle was obtained for each rat, as listed in the following table.
Theoretically, atrophy should be measured as the loss in weight of the muscle, but the initial weight of the muscle could not have been obtained without sacrificing the rat. Consequently, the initial total body weight X (in grams) of the rat was measured. It was assumed that this figure is closely related to the initial weight of the leg muscle.