Epidemiology: post-test probability: 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: post-test probability

Sample item

A patient has a pre-test probability of 20 percent for a disease. The positive likelihood ratio of the test is 6. What is the approximate post-test probability after a positive result?

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

The answer and the reasoning

B. 60 percent

Pre-test odds are 0.2 divided by 0.8, which is 0.25. Multiplying by the likelihood ratio of 6 gives post-test odds of 1.5, which equals a probability of 1.5 divided by 2.5, or 60 percent. Likelihood ratios work on odds, not on probabilities.

Option by option

  • A. 40 percent

    Forty percent would correspond to a likelihood ratio of about 2.7.

  • B. 60 percent (correct)

    Correct. Odds of 0.25 times 6 is 1.5, equal to 60 percent.

  • C. 80 percent

    Eighty percent would need a likelihood ratio of 16.

  • D. 30 percent

    Thirty percent would need a likelihood ratio of about 1.7.

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 B, 60 percent. Pre-test odds are 0.

Why is "40 percent" not the answer?

Forty percent would correspond to a likelihood ratio of about 2.7.

Why is "80 percent" not the answer?

Eighty percent would need a likelihood ratio of 16.

Why is "30 percent" not the answer?

Thirty percent would need a likelihood ratio of about 1.7.

What concept does this question test?

It tests Epidemiology: post-test probability in PSM. 2 divided by 0.8, which is 0.25. Multiplying by the likelihood ratio of 6 gives post-test odds of 1.5, which equals a probability of 1.5 divided by 2.5, or 60 percent. Likelihood ratios work on odds, not on probabilities.

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: post-test probability?

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.