Epidemiology: incidence rate ratio: 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 rate ratio

Sample item

In a cohort the incidence is 30 per 10,000 person-years in exposed workers and 6 per 10,000 person-years in unexposed workers. What is the incidence rate ratio?

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

The answer and the reasoning

B. 5

The incidence rate ratio is the incidence density in the exposed divided by that in the unexposed, 30 divided by 6, which is 5. The difference, 24 per 10,000 person-years, is the rate difference, a measure of excess risk. A ratio above 1 indicates a positive association.

Option by option

  • A. 1.2

    This is a calculation error that does not follow from the data.

  • B. 5 (correct)

    Correct. 30 divided by 6 gives a rate ratio of 5.

  • C. 24

    This is the rate difference, 30 minus 6, not the ratio.

  • D. 0.2

    This is the inverse, 6 divided by 30.

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, 5. The incidence rate ratio is the incidence density in the exposed divided by that in the unexposed, 30 divided by 6, which is 5.

Why is "1.2" not the answer?

This is a calculation error that does not follow from the data.

Why is "24" not the answer?

This is the rate difference, 30 minus 6, not the ratio.

Why is "0.2" not the answer?

This is the inverse, 6 divided by 30.

What concept does this question test?

It tests Epidemiology: incidence rate ratio in PSM. The difference, 24 per 10,000 person-years, is the rate difference, a measure of excess risk. A ratio above 1 indicates a positive association.

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 rate ratio?

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.