Epidemiology: incidence density: 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: incidence density

Sample item

In a cohort of 500 factory workers followed for unequal periods because of resignations and transfers, the total follow-up contributed is 400 person-years, during which 12 new cases of occupational asthma are detected. What is the incidence rate?

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

The answer and the reasoning

D. 30 per 1000 person-years

When follow-up periods differ, the incidence rate (incidence density) uses person-time at risk in the denominator. The calculation is 12 / 400 = 0.03 per person-year, or 30 per 1000 person-years. Using the number of people at the start gives a cumulative incidence that wrongly assumes everyone was followed for the full period.

Option by option

  • A. 3 per 1000 person-years

    This is an arithmetic slip of one decimal place; 12 / 400 is 0.03, not 0.003.

  • B. 24 per 1000 persons

    This divides by the 500 workers at the start (12 / 500) and ignores unequal follow-up, giving a risk rather than a rate.

  • C. 33 per 1000 person-years

    This is 400 / 12, the person-years per case, which is the reciprocal of the rate.

  • D. 30 per 1000 person-years (correct)

    Correct. New cases divided by total person-years at risk gives 30 per 1000 person-years.

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 D, 30 per 1000 person-years. When follow-up periods differ, the incidence rate (incidence density) uses person-time at risk in the denominator.

Why is "3 per 1000 person-years" not the answer?

This is an arithmetic slip of one decimal place; 12 / 400 is 0.03, not 0.003.

Why is "24 per 1000 persons" not the answer?

This divides by the 500 workers at the start (12 / 500) and ignores unequal follow-up, giving a risk rather than a rate.

Why is "33 per 1000 person-years" not the answer?

This is 400 / 12, the person-years per case, which is the reciprocal of the rate.

What concept does this question test?

It tests Epidemiology: incidence density in PSM. The calculation is 12 / 400 = 0.03 per person-year, or 30 per 1000 person-years. Using the number of people at the start gives a cumulative incidence that wrongly assumes everyone was followed for the full period.

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: incidence density?

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.