EXAMPLE 9 Testing insect repellants

Consumer Reports describes a method for comparing the effectiveness of two insect repellants. The active ingredient in one is 15% Deet. The active ingredient in the other is oil of lemon eucalyptus. Repellants are tested on several volunteers. For each volunteer, the left arm is sprayed with one of the repellants and the right arm with the other. This is a matched pairs design in which each subject compares two insect repellants. To guard against the possibility that responses may depend on which arm is sprayed, which arm receives which repellant is determined randomly. Beginning 30 minutes after applying the repellants, once every hour, volunteers put each arm in separate 8-cubic-foot cages containing 200 disease-free female mosquitoes in need of a blood meal to lay their eggs. Volunteers leave their arms in the cages for five minutes. The repellant is considered to have failed if a volunteer is bitten two or more times in a five-minute session. The response is the number of one-hour sessions until a repellant fails.