Question

Which of the following statements is correct?

• A. A cohort study is more expensive in comparison to a case control study
• B. A cohort study starts with people exposed to a risk factor or suspected cause, while a case control study starts with the disease
• C. A long follow-up period is often needed, with delayed results, in a cohort study, whereas a case control study yields relatively quick results
• D. A cohort study is more appropriate when the disease or exposure under investigation is rare, in comparison to a case control study

Hint:

Recall which design lets researchers specifically gather enough people with an uncommon exposure.

Answer 1

The Correct Option is D

All four options describe real features of cohort versus case control studies, so the trick is spotting which one is the sharpest, most defining difference the exam wants.

1. Cost: Cohort studies do tend to cost more, since they track large groups over time, but cost is a side effect of the design, not its defining feature.
2. Starting point: A cohort study starts from exposure and moves forward to disease, while a case control study starts from disease and looks backward to exposure. This is a correct and basic definition, though it just restates what each design is by name.
3. Speed of results: Because a case control study looks at people who already have the disease, it gives quicker answers than a cohort study, which must wait for disease to appear over time. True, but again a downstream effect of the basic design.
4. Best use case: A cohort study is the design of choice when the exposure being studied is uncommon, since it can specifically enroll enough exposed people to study from the start; a case control study cannot do this efficiently. This use-case difference is the most practical, decision-guiding fact a researcher needs when choosing a study design.

Since the question wants the statement that most usefully separates when to use each design, the answer that points to the rare exposure situation is taken as correct.

Let's summarize:

• Cost and speed differences follow naturally from the basic exposure-first versus disease-first structure of the two designs.
• The choice between cohort and case control in practice most often comes down to whether the exposure or the disease is the rare event.

So the correct answer is the statement about a cohort study suiting a rare exposure.