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

Step 1: Understanding the Question.
This question compares cohort studies and case control studies and asks which single statement best describes a real difference between the two designs.

Step 2: Key Concept.
In a cohort study, we start by picking a group already exposed to a risk factor and a group not exposed, then follow both forward in time to see who develops the disease. In a case control study, we start with people who already have the disease (cases) and compare their past exposure with people who do not have the disease (controls). This basic direction of the two designs decides most of their other differences, including cost, time taken, and which situations suit each design best.

Step 3: Detailed Explanation.
A cohort study usually costs more than a case control study, since it needs a large group followed over time, often with repeated visits and tests, so this statement is true in general.
The definition given for direction of the two study types, starting from exposure in a cohort versus starting from disease in a case control study, is exactly correct and is the core textbook distinction between them.
A cohort study often does need a long follow-up before enough cases build up to analyze, while a case control study can be finished quickly since the disease has already happened by the time we start; this statement about the pace of each design is also true.
A cohort study is well suited to a rare exposure, because it deliberately gathers a full group of exposed people from the start, something a case control study cannot easily do since it works backward from disease. This is the classic reason a cohort design is chosen over a case control design when the exposure itself is uncommon, and it is the sharpest, most specific difference among the choices given, since a case control study is usually chosen instead when it is the disease, not the exposure, that is rare.

Step 4: Final Answer.
The statement that best captures why researchers pick a cohort study over a case control design, its strength with a rare exposure, is the one taken as correct.