Case-control studies: advantages and disadvantages

Researchers investigated the risk factors associated with the development of pulmonary tuberculosis in Russia. A case-control study was performed in the city of Samara, 700 miles south east of Moscow. Cases were 334 consecutive adults diagnosed as having culture confirmed pulmonary tuberculosis at any of the city’s specialist tuberculosis clinics between 1 January 2003 and 31 December 2003. For each case, a control matched for year of birth and sex, and with no history of tuberculosis, was sampled randomly from a registry of the general population of Samara city. A questionnaire was used to collect information retrospectively about potential risk factors before and during the development of pulmonary tuberculosis. Controls were asked about exposure to risk factors before the index date for their matched case—that is, the date when tuberculosis was diagnosed.1

The researchers reported that the most important risk factors associated with the development of pulmonary tuberculosis were raw milk and unemployment.

Which of the following statements, if any, are true?

• a) The sampling of the controls was prone to selection bias

• b) The information collected by the questionnaire was prone to recall bias

• c) It was possible to estimate the population at risk of pulmonary tuberculosis

• d) It can be inferred that raw milk and unemployment cause pulmonary tuberculosis