Which of the following is a disadvantage of cohort studies?

• A. They require randomized exposure assignment
• B. They are quick and inexpensive to conduct
• C. Retrospective designs require rigorous registries
• D. They provide instantaneous results

Answer: (C)

The main idea tested is how dependent retrospective cohort studies are on the quality of existing data to determine exposure and outcome. When registries or records are rigorous and complete, the study can be valid, but if the data are poor or incomplete, misclassification of exposure or outcome, missing data, and bias can seriously undermine findings. This makes the requirement for high-quality registries a real disadvantage of retrospective designs. In contrast, cohort studies are observational and do not involve randomized exposure assignment; they are not typically quick or inexpensive, and results are not instantaneous, since follow-up over time is needed to observe outcomes.