Demography: Pearl index: 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 · Demography: Pearl index

Sample item

In a trial of an oral contraceptive, 200 women are followed for 2 years and 5 pregnancies occur while the pill is being used correctly. What is the Pearl index?

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

The answer and the reasoning

B. 1.25 per 100 woman-years

The Pearl index is the number of accidental pregnancies per 100 woman-years of exposure. Exposure here is 200 x 2 = 400 woman-years. Then 5 / 400 x 100 = 1.25 per 100 woman-years.

Option by option

  • A. 2.5 per 100 woman-years

    This divides by 200 women and ignores the second year of follow-up.

  • B. 1.25 per 100 woman-years (correct)

    Correct. 5 / 400 x 100 = 1.25.

  • C. 12.5 per 100 woman-years

    This expresses the rate per 1,000 woman-years but labels it per 100.

  • D. 80 per 100 woman-years

    This is the number of woman-years per pregnancy, which is the reciprocal of the rate.

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 B, 1.25 per 100 woman-years. The Pearl index is the number of accidental pregnancies per 100 woman-years of exposure.

Why is "2.5 per 100 woman-years" not the answer?

This divides by 200 women and ignores the second year of follow-up.

Why is "12.5 per 100 woman-years" not the answer?

This expresses the rate per 1,000 woman-years but labels it per 100.

Why is "80 per 100 woman-years" not the answer?

This is the number of woman-years per pregnancy, which is the reciprocal of the rate.

What concept does this question test?

It tests Demography: Pearl index in PSM. Exposure here is 200 x 2 = 400 woman-years. Then 5 / 400 x 100 = 1.25 per 100 woman-years.

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 Demography: Pearl index?

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.