To measure the effect of exercise on the risk of heart disease, investigators compared the incidence of this disease for two large groups of London Transport Authority busmen-drivers and conductors. The conductors got a lot more exercise as they walked around all day collecting fares.

The age distributions for the two groups were very similar, and all the subjects had been on the same job for 10 years or more. The incidence of heart disease was substantially lower among the conductors, and the investigators concluded that exercise prevents heart disease.

Other investigators were skeptical. They went back and found that London Transport Authority had issued uniforms to drivers and conductors at the time of hire; a record had been kept of the sizes.

(a) Why does it matter that the age distributions of the two groups were similar?

(b) Why does it matter that all the subjects had been on the job for 10 years or more?

(c) Why did the first group of investigators compare the conductors to drivers, not to London Transport Authority executive staff?

(d) Why might the second group of investigators have been skeptical?

(e) What would you do with the sizes of the uniforms? P-698