A researcher is interested in studying whether there is an association between neural tube defects and use of acetaminophen during the first 3 months of pregnancy. He randomly chooses a group of women who have just delivered babies with neural tube defects, and a second group of women who delivered apparently healthy babies. These 2 groups were then asked about their use of acetaminophen during the first 3 months of pregnancy. Which of the following measures of association are the investigators most likely to report?

Question

A researcher is interested in studying whether there is an association between neural tube defects and use of acetaminophen during the first 3 months of pregnancy.  He randomly chooses a group of women who have just delivered babies with neural tube defects, and a second group of women who delivered apparently healthy babies.  These 2 groups were then asked about their use of acetaminophen during the first 3 months of pregnancy.  Which of the following measures of association are the investigators most likely to report?

Quizplus · Accepted Answer

This scenario describes a typical case-control study design. People with the disease of interest (ie, cases [women who have just delivered babies with neural tube defects]) and people without this disease (ie, controls [women who delivered apparently healthy babies]) are asked about previous exposure to the risk factor being studied (eg, acetaminophen use during the first 3 months of pregnancy). The main measure of association is the odds ratio (OR). The OR can be calculated as follows: OR = (odds of exposure in cases) / (odds of exposure in controls) (Choice A) Median survival is calculated in cohort studies or clinical trials, and is usually used to compare the median survival times in >2 groups of patients (eg, those receiving a new treatment and those receiving a placebo). (Choice C) Prevalence ratio is a ratio of 2 probabilities (ie, prevalence) commonly used in cross-sectional studies to compare the prevalence of disease among exposed and unexposed groups of people. In case-control studies, cases and controls are selected from a pool of potential cases and controls that do not represent the real frequency (ie, prevalence) of disease in the population. Therefore, estimating the prevalence of disease in case-control studies would be biased. (Choices D and E) Incidence measures (eg, relative risk or relative rate) cannot be directly measured in case-control studies because cases, by definition, already have the disease (ie, the study does not follow a cohort for development of a disease). Relative risk and relative rate are calculated in prospective and retrospective cohort studies, where risk factor assessment occurs first and then exposed and unexposed groups are followed over time and assessed for development of disease. Educational objective: A case-control study is used to compare the exposure status of people with the disease (ie, cases) to the exposure status of people without the disease (ie, controls). The main measure of association is the odds ratio.

A Researcher Is Interested in Studying Whether There Is an Association