A study is conducted to determine the efficacy of influenza vaccine. Subjects agree to participate for 2 years. During the first year, subjects are randomly assigned to be injected with either an inert substance (a placebo) or the active vaccine. During the second year, each subject who previously received the placebo is given the active vaccine, and each subject who previously received the active vaccine is given the placebo. Each subject serves as his or her own control. All incident cases of influenza are recorded, and the occurrence of influenza when vaccinated is compared with the occurrence when unvaccinated. The appropriate test of significance for this study is the