A clinical team studies whether evening caffeine intake affects sleep duration. Prior evidence suggests that alcohol use and stress level also influence sleep. There is also unmeasured genetic variability in caffeine metabolism among people that cannot currently be identified. The researchers plan a completely randomized experiment in which participants are assigned to a high-caffeine-evening group or a no-caffeine-evening group. For each factor listed (evening caffeine intake, alcohol use, stress level, genetic metabolism variability) determine whether the factor can be controlled or cannot be controlled. If a factor cannot be controlled, state what should be done to reduce variability in the sleep-duration response.
