An investigator designs a study to estimate the incidence of type 2 diabetes in an urban slum over 5 years by repeatedly examining the same cohort annually. At baseline, all participants are diabetes-free. What is the key advantage of this design over a case-control study for this purpose?

• A It can establish temporal sequence of exposure before disease ✓
• B It is faster and cheaper than a case-control study
• C It is the best design for studying rare diseases
• D It eliminates recall bias by design

Explanation

The defining advantage of a prospective cohort study is that exposure status is measured before disease onset, establishing temporal sequence — a requirement for causation. Case-control studies are retrospective and subject to recall bias in establishing exposure-disease sequence. Cohort studies are more expensive and time-consuming than case-control studies (negating option B). Rare diseases are best studied with case-control designs (not cohort — option C). While prospective cohort studies reduce recall bias for exposure, this is a secondary benefit; the primary one is temporal precedence.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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