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

A cohort study follows 300 persons for varying periods, contributing a total of 600 person-years. During follow-up 12 new cases of the disease occur. What is the incidence density?

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

The answer and the reasoning

D. 2 per 100 person-years

Incidence density (incidence rate) is the number of new cases divided by the total person-time at risk: 12 / 600 = 0.02, or 2 per 100 person-years. It is used when follow-up differs between participants. Dividing by the number of persons instead gives a cumulative incidence (risk) of 4%.

Option by option

  • A. 12 per 100 person-years

    12 per 100 person-years ignores the actual denominator of 600 person-years.

  • B. 50 per 100 person-years

    600 / 12 inverts the ratio and gives person-years per case.

  • C. 4 per 100 persons

    12 / 300 = 4% is the proportion who developed disease at some time in follow-up (cumulative incidence), not a rate per person-time.

  • D. 2 per 100 person-years (correct)

    Dividing new cases by person-time at risk gives 2 per 100 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.

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Questions about this PSM question

What is the answer to this PSM question?

The best answer is D, 2 per 100 person-years. Incidence density (incidence rate) is the number of new cases divided by the total person-time at risk: 12 / 600 = 0.

Why is "12 per 100 person-years" not the answer?

12 per 100 person-years ignores the actual denominator of 600 person-years.

Why is "50 per 100 person-years" not the answer?

600 / 12 inverts the ratio and gives person-years per case.

Why is "4 per 100 persons" not the answer?

12 / 300 = 4% is the proportion who developed disease at some time in follow-up (cumulative incidence), not a rate per person-time.

What concept does this question test?

It tests Epidemiology: incidence density in PSM. 02, or 2 per 100 person-years. It is used when follow-up differs between participants. Dividing by the number of persons instead gives a cumulative incidence (risk) of 4%.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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.