Table 30 shows the mean changes in weight (in pounds) by people who took part in one of four popular diets. Forty people were randomly assigned to each of the diets. The mean changes were found after 2 months, 6 months, and 12 months.

a. Which of the four groups of people had the largest mean decrease in weight after 2 months?
b. Which of the four groups of people had the largest mean decrease in weight after 12 months?
c. For each diet, compare the mean change in weight after 2 months with the mean change in weight after 12 months. What do these comparisons mean in this situation?
d. For the first 2 months, the researchers asked the participants to follow their diet plan to the best of their ability. But after that, the researchers encouraged the participants to follow their diet as much or little as they wanted. Explain how this might explain what you observed in part (c). What else might be the reason?
e. The standard deviations of the changes in weight (in pounds) of the participants are shown in Table 31.

A student concludes that almost all the Ornish participants lost more weight than the Zone participants after 12 months, because the mean decrease in weight was larger for the Ornish participants. What would you tell the student, considering that the standard deviations shown in Table 31 are relatively large?

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Table 30 Participants' Mean Changes in Weight by Four Popular Diets Mean Change in Weight (in pounds) Duration Atkins Zone Weight WatchersOrnish® -3.6 3.8 2 months 6 months -32 12 months 2.1 -3.5 -3.5 -3.0 -3.6 3.6 -3.3 -3.2 Table 31 Standard Deviations of Changes in Weight by Four Popular Diets Standard Deviation of the Change in Weight (in pounds) Duration k Zone Weight Watchers Ornish 2 months 6 months 12 months 3.3 4.9 4.8 3.6 5.7 6.0 3.8 5.6 4.9 3.4 6.7 7.3