Submit test An experiment investigates a dietary approach to treating insomnia. Researchers randomly assigned 29 subjects to two treatment groups, one taking a high dose of calcium supplements and the other a placebo. Suppose some of the 29 subjects were very active people who walked at a lot or got vigorous exercise several times a week, while others tended to be more sedentary, working office jobs and watching a lot of TV. Why might researchers choose to block the subjects by activity level before randomly assigning them to the calcium supplements and placebo groups?

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