Exercise 24 describes an experiment showing that exercise helped people sleep better. The experiment involved other groups of subjects who didn’t exercise. Why didn’t the experimenters just have everyone exercise and see if their ability to sleep improved?

Data From Exercise 24

Is diet or exercise effective in combating insomnia? Some  believe that cutting out desserts can help alleviate the problem,  while others recommend exercise. Forty volunteers suffering  from insomnia agreed to participate in a month-long test. Half  were randomly assigned to a special no-desserts diet; the others  continued desserts as usual. Half of the people in each of these  groups were randomly assigned to an exercise program, while  the others did not exercise. Those who ate no desserts and  engaged in exercise showed the most improvement.