2.27 Let D denote having a certain disease and E denote having exposure to a certain risk factor. The attributable risk (AR) is the proportion of disease cases attributable to that exposure (see Benichou 2005).

a. Let P(E) = 1 - P(E). Explain why AR = [P(D) – P(D|E)]/P(D).

b. Show that AR relates to the relative risk RR by AR = [P(E)(RR – 1)]/[1 + P(E)(RR – 1)].