The investigators decided to select controls from office records of physicians who admitted the cases and to enroll two controls per case. A total of 28 cases and 56 controls were enrolled in the case-control study. Cases and controls were asked about exposures to cooling towers and nearby buildings.

Of the 28 cases, 3 reported having visited a particular hospital with a cooling tower; on the other hand, of the 56 controls, 7 reported having visited that hospital (Hospital B). Similarly, 7 of the cases reported having visited the post office (1 was unsure), and 12 of the controls reported having visited the post office (6 were unsure).

• QUESTION 11:
  Calculate the odds ratios for illness with Legionnaires' disease among persons who visited these two locations. Why might the numbers of cases and controls included in these two calculations differ? What is your interpretation of the results?