EXAMPLE 3.26 Matched Pairs for the Cell Phone Experiment

Example 3.22 (page 151) describes an experiment on the effects of talking on a cell phone while driving. The experiment compared two treatments: driving in a simulator and driving in a simulator while talking on a hands-free cell phone. The response variable is the time the driver takes to apply the brake when the car in front brakes suddenly. In Example 3.22, 40 student subjects were assigned at random, 20 students to each treatment. Subjects differ in driving skill and reaction times. The completely randomized design relies on chance to create two similar groups of subjects.

In fact, the experimenters used a matched pairs design in which all subjects drove under both conditions. They compared each subject’s reaction times with and without the phone. If all subjects drove first with the phone and then without it, the effect of talking on the cell phone would be confounded with the fact that this is the first run in the simulator. The proper procedure requires that all subjects first be trained in using the simulator, that the order in which a subject drives with and without the phone be random, and that the two drives be on separate days to reduce the chance that the results of the second treatment will be affected by the first treatment.