Epidemiology: population attributable fraction: 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: population attributable fraction

Sample item

In a community, 30% of adults smoke. The relative risk of chronic bronchitis in smokers compared with non-smokers is 5. Using Levin's formula, what proportion of all chronic bronchitis in this community is attributable to smoking?

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

The answer and the reasoning

D. About 55%

Levin's formula gives the population attributable fraction as p(RR - 1) / [1 + p(RR - 1)], where p is the prevalence of exposure. Here p(RR - 1) = 0.3 x 4 = 1.2, so the fraction is 1.2 / 2.2 = 0.545, or about 55%. It is the share of disease in the whole community that would disappear if the exposure were removed.

Option by option

  • A. About 80%

    (RR - 1) / RR = 4 / 5 = 80% is the attributable fraction among the exposed only, not in the whole community.

  • B. About 30%

    30% is simply the prevalence of smoking and ignores the relative risk.

  • C. About 24%

    Multiplying exposure prevalence by the attributable fraction in the exposed (0.3 x 0.8) is not the formula for the population attributable fraction.

  • D. About 55% (correct)

    The calculation 1.2 / 2.2 gives about 55%, the share of all cases in the community due to smoking.

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.

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Questions about this PSM question

What is the answer to this PSM question?

The best answer is D, About 55%. Levin's formula gives the population attributable fraction as p(RR - 1) / [1 + p(RR - 1)], where p is the prevalence of exposure.

Why is "About 80%" not the answer?

(RR - 1) / RR = 4 / 5 = 80% is the attributable fraction among the exposed only, not in the whole community.

Why is "About 30%" not the answer?

30% is simply the prevalence of smoking and ignores the relative risk.

Why is "About 24%" not the answer?

Multiplying exposure prevalence by the attributable fraction in the exposed (0.3 x 0.8) is not the formula for the population attributable fraction.

What concept does this question test?

It tests Epidemiology: population attributable fraction in PSM. Here p(RR - 1) = 0.3 x 4 = 1.2, so the fraction is 1.2 / 2.2 = 0.545, or about 55%. It is the share of disease in the whole community that would disappear if the exposure were removed.

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: population attributable fraction?

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.