Multiple _Choice Questions and answers. Exam (elaborations) NR503

When does confounding occur? A. When the characteristics of the participants in the exposed group differ in some important way from those in the unexposed group B. When the characteristics of the participants in the study sample differ in some important way from those in the source population C. When the controls are not matched to the cases in a case–control study D. When the people lost to follow‐up in a cohort study differ from those who are not lost to follow‐up E. It occurs in all epidemiological research Answer: A. Confounding will occur when the exposed and unexposed differ in terms of some other characteristic that is associated with the outcome of interest, i.e. the groups are not exchangeable. While a lack of matching in a case‐control study may lead to confounding this is not inevitable (C), while (B) and (D) are issues of selection bias. And although confounding can occur in all observational epidemiological research, it should not be a problem in large randomised controlled trials (E). 2. What is the best single way to prevent confounding from affecting the results of a study? A. Randomisation B. Restriction C. Stratification D. Matching E. Multivariable analysis Answer: A. Randomisation is the best way to prevent confounding as this will maximise the chances that all confounders (known and unknown) are balanced across the study groups, thereby maximising their exchangeability. While the other options offer approaches to managing confounding that can be very effective in particular situations, they can only account for known confounders. 3. In a study of alcohol and oral cancer the crude relative risk is 4.0 overall but it is 2.0 for men and 2.0 for women when the sexes are considered separately. This suggests that: A. There is confounding by sex in these data B. There is confounding by some unknown or unmeasured factor in these data C. There is evidence of effect modification by sex in these data D. The results have been adjusted for age and sex E. The results are due to bias Answer: A. There is confounding by sex in these data. The crude relative risk (RR) is 4.0 but when we stratify by sex the RR is 2.0 for both men and women. When the stratum‐