Researchers want to estimate the association between environmental lead exposure and cognitive deficits in children. Among all children who received preventive care at 5 local pediatric clinics over the last 5 years, they identify 20 children diagnosed with cognitive deficits and 40 without the diagnosis. The researchers then review the patients' medical records and calculate the distance between the patients' residence and known lead-contaminated geographic areas. Which of the following best describes the study design used by the researchers?
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Observational studies (eg, case series, case-control studies, cohort studies, cross-sectional studies) differ from experimental studies (eg, clinical trials) in that the researchers passively observe the study participants rather than monitor the result of a specific intervention. Observational studies can be subdivided as follows:
Descriptive studies collect data to characterize a specific health problem (eg, disease prevalence, incidence rates).
Analytical studies test hypotheses to evaluate associations between risk factors and disease.
A case-control study is an analytical observational design in which potential participants are initially identified as "cases" or "controls" according to an outcome status (ie, disease of interest). In this example, children diagnosed with cognitive deficits are cases and children without cognitive deficits are controls. Once identified, cases and controls are then assessed retrospectively for exposure to risk factors of interest. In this case, the risk factor of interest is environmental lead exposure, as measured by the distance between the patients' residence and known lead-contaminated areas. Finally, the prevalence of exposure to the risk factors (ie, frequency of exposure) is compared between cases and controls to estimate the association between the risk factors and the outcomes.
(Choice B) A case series study is a descriptive study that tracks patients with a known condition (eg, exposure, risk factor, disease) to document the natural history or response to treatment. Unlike a case-control study, a case series is purely descriptive and cannot establish associations.
(Choice C) Cross-sectional studies may evaluate associations between risk factors and outcomes of interest. However, unlike in case-control studies (participants selected based on the outcome status), the participants in a cross-sectional study are selected based on the inclusion and exclusion criteria set for the study.
(Choice D) In cohort studies, 2 groups of individuals (ie, cohorts) are initially identified based on their exposure status to a specific risk factor (eg, environmental lead exposure) rather than based on their outcome status (eg, cognitive deficits), as seen in this example. The cohorts are then followed over time for development of the outcome.
(Choice E) A randomized clinical trial would follow individuals who have been randomized to either a treatment arm or a control arm and compare the effect of the intervention.
Educational objective:
A case-control study is an observational study design; it begins with selecting individuals who have the outcome ("cases") and individuals who do not have the outcome ("controls") and then retrospectively comparing their history of exposure to risk factors.