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Difficulty: HardEpidemiologic Study Designs

Investigators at a university hospital are evaluating the relationship between severe obstructive sleep apnea (OSA) and the subsequent development of atrial fibrillation. In January 2026, the team queries a comprehensive national electronic health registry. They define an exposed group by identifying 15,000 adult patients who received a new polysomnography-confirmed diagnosis of severe OSA between January 2012 and December 2015. They simultaneously identify an unexposed group of 30,000 patients without an OSA diagnosis during the same time frame, matched for age, biologic sex, and body mass index. The researchers then extract longitudinal data, reviewing each patient's medical records from their index date through December 2025 to calculate the incidence of newly diagnosed atrial fibrillation in both groups. Which of the following is the most appropriate primary statistical measure of association for this study?

  1. A
    Odds ratio
  2. B
    Prevalence ratio
  3. C
    Correlation coefficient
  4. Relative riskAnswer
  5. E
    Standardized mortality ratio

Answer

Relative risk
The correct answer is relative risk. The scenario describes a retrospective cohort study. The investigators defined the study groups based on their exposure status (presence or absence of obstructive sleep apnea) in the past (2012-2015) and then followed them longitudinally through historical medical records to ascertain the outcome (atrial fibrillation) up to 2025. Because cohort studies track patients from exposure to outcome, they can directly measure the true incidence of the disease. Therefore, the most appropriate measure of association is relative risk (risk ratio), which compares the incidence of the outcome in the exposed group to the incidence in the unexposed group.

Step-by-Step Solution

1
Identify the sampling strategy.
Patients were selected based on their exposure status (OSA vs. no OSA) between 2012 and 2015.
Determining how groups are initially formed is the first step in identifying the epidemiologic study design.
2
Analyze the temporal relationship between data collection and events.
The researchers looked forward in time from the index date up to 2025 to determine the incidence of atrial fibrillation, relying on historical medical records.
Establishing the timeline differentiates between retrospective cohort, prospective cohort, and case-control studies.
3
Determine the appropriate statistical measure for the identified design.
The study is a retrospective cohort study, meaning the appropriate primary measure of association is relative risk (risk ratio).
Cohort studies directly measure the incidence of disease in exposed and unexposed groups, allowing for the calculation of relative risk.

Key Concept

Retrospective cohort studies identify groups based on past exposure status and track them longitudinally using historical data to calculate disease incidence and relative risk.
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Epidemiologic Study Designs

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