Name four major study designs and a key strength of each.

• A. Cohort: incidence and temporality; Case-control: efficient for rare outcomes; Cross-sectional: assesses prevalence and associations at a single time; Ecologic: population-level analyses; quick, generates hypotheses.
• B. Cohort: cross-sectional prevalence; Case-control: randomization; Cross-sectional: incidence over time; Ecologic: individual-level data.
• C. Cohort: quick, cheap; Case-control: temporality; Cross-sectional: incidence; Ecologic: experimental evidence.
• D. Cohort: blinding; Case-control: randomization; Cross-sectional: longitudinal; Ecologic: cohort analysis.

Answer: (A)

Understanding the strengths of common study designs helps you match the design to the question you’re asking. A cohort study shines because it follows people over time, allowing you to observe whether exposure leads to an outcome and to estimate incidence and relative risk, since temporality is preserved. A case-control design is especially efficient for studying rare outcomes: you start with people who have the outcome and compare their past exposures to those without the outcome, making it resource-friendly for rare diseases. A cross-sectional study gives a snapshot of a population at one point in time, so it’s ideal for measuring how common a condition is (prevalence) and for spotting associations, though it can’t establish which came first. An ecologic study looks at data at the population level, which helps detect broad patterns across groups or regions and can generate hypotheses with relatively low cost and data requirements. The other options mix up these strengths—for example, assigning randomization to case-control, claiming cross-sectional can track incidence over time, or suggesting ecologic studies provide detailed individual-level data—so they don’t fit how these designs truly function.