Question

A retrospective cohort study is examining birth complications in women with diabetes. The study determines that babies are more likely to be born large for gestational age (LGA) if the mother has diabetes. The relative risk for the study is calculated to be 4. Which of the fol- lowing accurately describes this relative risk?

1. (A)  The incidence rate of diabetes among mothers with LGA babies is four times that of non-LGA mothers

2. (B)  The incidence rate of LGA among women with diabetes is four times that of women without diabetes

3. (C)  The incidence rate of LGA among women without diabetes is four times that of women with diabetes

4. (D)  The odds of diabetes among mothers with LGA babies is four times that of non-LGA mothers

5. (E)  The odds of LGA among women with dia- betes is four times that of women without diabetes

Answer 1

The correct answer is B. A retrospective co- hort study includes a group of subjects who had a certain condition or received a certain treatment at some time in the past and com- pares their outcomes to those of another group (a control group) made up of subjects who did not have this condition or receive the treat- ment. In this study the risk factor is the pres- ence of diabetes in the mothers and the out- come is LGA babies. The incidence of LGA births in women with diabetes is four times that in women without diabetes. Relative risk is

defined as the incidence rate of some outcome in those exposed to a risk factor divided by the incidence rate of those not exposed. This defi- nition gives the factor at which the incidence rate of LGA among women with diabetes is larger than the incidence rate of LGA among women without diabetes.

Answer A is incorrect. This choice describes the correct type of risk analysis but describes the relationship in reverse.

Answer C is incorrect. This choice reverses the findings of the study, which shows that the incidence of LGA is four times more in women with diabetes.

Answer D is incorrect. This choice incorrectly uses odds rather than incidence rates and also describes the relationship of the findings of the study in reverse.

Answer E is incorrect. This choice describes an odds ratio for a case-control study. A case- control study evaluates the presence of risk factors in people with and without a disease. Although this is the opposite of a cohort study, the results are still reported in terms of disease presence with respect to risk factors; that is, the presence or absence of disease is categorized in the group with risk factors and compared to the group without risk factors. The difference, however, is that odds are used rather than inci- dence. The incidence rate is a percentage (eg, 50 out of 100). Odds are calculated by dividing those with disease by those without (50 to 50, or 1 to 1).