Which of the following is an observational study design?

• A. Randomized trial
• B. Cohort study
• C. Laboratory experiment
• D. Systematic review

Answer: (B)

Observational studies are those in which you observe exposures and outcomes without actively assigning or controlling who receives a exposure. A cohort study fits this because you identify a group of people based on their exposure status and then follow them over time to see who develops the outcome, all without the researcher deciding who is exposed. This design can be prospective or retrospective, and it allows estimation of incidence and measures like risk ratios.

In contrast, a randomized trial assigns the exposure randomly to participants, making it an experimental design. A laboratory experiment also involves deliberate manipulation in a controlled setting. A systematic review compiles results from existing studies rather than collecting new data from participants, so it isn’t a primary observational study.