Epidemiology: effect modification: 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: effect modification

Sample item

In a cohort, the crude relative risk of lung cancer with smoking is 3.0. Stratified by asbestos exposure, the relative risks are 10 in the exposed and 1.2 in the unexposed. Which phenomenon explains this?

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

The answer and the reasoning

B. Effect modification

When the strength of an association differs across strata of a third variable, that variable is an effect modifier, and stratum-specific estimates should be reported separately. In confounding the stratum-specific estimates are similar and differ from the crude one. Effect modification is a real biological interaction, not a bias.

Option by option

  • A. Confounding

    Confounding would give similar stratum-specific values that differ from the crude value.

  • B. Effect modification (correct)

    Correct. Stratum-specific estimates differ markedly, so asbestos modifies the effect.

  • C. Information bias

    Information bias comes from errors in measuring exposure or outcome.

  • D. Selection bias

    Selection bias arises from how subjects enter the study, not from stratified results.

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.

More PSM questions

Questions about this PSM question

What is the answer to this PSM question?

The best answer is B, Effect modification. When the strength of an association differs across strata of a third variable, that variable is an effect modifier, and stratum-specific estimates should be reported separately.

Why is "Confounding" not the answer?

Confounding would give similar stratum-specific values that differ from the crude value.

Why is "Information bias" not the answer?

Information bias comes from errors in measuring exposure or outcome.

Why is "Selection bias" not the answer?

Selection bias arises from how subjects enter the study, not from stratified results.

What concept does this question test?

It tests Epidemiology: effect modification in PSM. In confounding the stratum-specific estimates are similar and differ from the crude one. Effect modification is a real biological interaction, not a bias.

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: effect modification?

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.