Epidemiology: odds ratio from case-control study: practice question

A PSM question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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PSM · Epidemiology: odds ratio from case-control study

Sample item

A case-control study of a rare childhood tumour reports an odds ratio of 3.2, which is quoted as the relative risk. Why is this acceptable?

AThe sample was largeBThe controls were matchedCThe disease is rare, so the odds ratio approximates the relative riskDThe confidence interval excluded 1

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. The disease is rare, so the odds ratio approximates the relative risk

When the disease is rare, the odds ratio from a case-control study closely approximates the relative risk, since cases are a small fraction of the exposed and unexposed. Matching, sample size and significance do not provide this.

Option by option

A. The sample was large
Sample size does not change this relationship.
B. The controls were matched
Matching controls does not convert odds to risk.
C. The disease is rare, so the odds ratio approximates the relative risk (correct)
Correct. Rarity makes the OR approximate RR.
D. The confidence interval excluded 1
Significance does not make OR equal RR.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is C, The disease is rare, so the odds ratio approximates the relative risk. When the disease is rare, the odds ratio from a case-control study closely approximates the relative risk, since cases are a small fraction of the exposed and unexposed.

Why is "The sample was large" not the answer?

Sample size does not change this relationship.

Why is "The controls were matched" not the answer?

Matching controls does not convert odds to risk.

Why is "The confidence interval excluded 1" not the answer?

Significance does not make OR equal RR.

What concept does this question test?

It tests Epidemiology: odds ratio from case-control study in PSM. Matching, sample size and significance do not provide this.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Epidemiology: odds ratio from case-control study?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.