Answer (a) — incorrect: While it is true that the greatest number of Star personnel reside in Area D, Area D does not have the greatest overall number of SARS cases. Proportionally, Area D has a lower cumulative incidence of SARS than one other area in Epiville.

Answer (b) — incorrect: Area B has the largest population and therefore, the largest population at risk for developing SARS. Cumulative incidence is, however, defined as the number of new cases divided by the number of persons at risk during a specified time period. Therefore, knowing the number of persons at risk is not enough to conclude that the cumulative incidence is highest in area B.

Answer (c) — correct: Cumulative incidence is a function of both the number of new cases and the number of people at risk. You will get the greatest cumulative incidence when your numerator (number of new cases) is large and/or when the denominator (number of people at risk) is small. Since the greatest number of cases happens to have occurred in the Amoy Apartment complex that is located in Area B, the cumulative incidence is highest in this area as well (1.32 per 1,000 per three-week period).