Epidemiology: number needed to harm: 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.

Try it

PSM · Epidemiology: number needed to harm

Sample item

In a trial, serious bleeding occurred in 3 percent of patients on a drug and 1 percent on placebo. How many patients must be treated to cause one additional serious bleed?

A33B50C2D100

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

The answer and the reasoning

B. 50

The absolute risk increase is 3 minus 1, or 2 percent, which is 0.02. The number needed to harm is 1 divided by 0.02, which is 50. It is the counterpart of the number needed to treat.

Option by option

A. 33
Thirty-three would result from using the drug risk of 3 percent alone.
B. 50 (correct)
Correct. NNH is the reciprocal of the absolute risk increase of 0.02.
C. 2
Two is the absolute risk increase in percentage points and not its reciprocal.
D. 100
One hundred would result from using the placebo risk of 1 percent alone.

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.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is B, 50. The absolute risk increase is 3 minus 1, or 2 percent, which is 0.

Why is "33" not the answer?

Thirty-three would result from using the drug risk of 3 percent alone.

Why is "2" not the answer?

Two is the absolute risk increase in percentage points and not its reciprocal.

Why is "100" not the answer?

One hundred would result from using the placebo risk of 1 percent alone.

What concept does this question test?

It tests Epidemiology: number needed to harm in PSM. 02. The number needed to harm is 1 divided by 0.02, which is 50. It is the counterpart of the number needed to treat.

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: number needed to harm?

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.