Biostatistics: negative likelihood 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 · Biostatistics: negative likelihood ratio

Sample item

A rapid test for a condition has a sensitivity of 80% and a specificity of 90%. What is its negative likelihood ratio?

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

The answer and the reasoning

D. 0.22

The negative likelihood ratio is (1 - sensitivity)/specificity = 0.20/0.90 = 0.22. It shows how much a negative result lowers the odds of disease, and the smaller the value the more useful a negative result is. The positive likelihood ratio, sensitivity/(1 - specificity), would be 0.80/0.10 = 8.

Option by option

  • A. 8

    This is the positive likelihood ratio, 0.80/0.10.

  • B. 0.11

    This is 0.10/0.90, which uses the false positive rate (1 - specificity) in the numerator instead of the false negative rate.

  • C. 4.5

    This is the inverse (0.90/0.20), which has no standard meaning here.

  • D. 0.22 (correct)

    Correct. 0.20 / 0.90 = 0.22.

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, 0.22. The negative likelihood ratio is (1 - sensitivity)/specificity = 0.

Why is "8" not the answer?

This is the positive likelihood ratio, 0.80/0.10.

Why is "0.11" not the answer?

This is 0.10/0.90, which uses the false positive rate (1 - specificity) in the numerator instead of the false negative rate.

Why is "4.5" not the answer?

This is the inverse (0.90/0.20), which has no standard meaning here.

What concept does this question test?

It tests Biostatistics: negative likelihood ratio in PSM. 20/0.90 = 0.22. It shows how much a negative result lowers the odds of disease, and the smaller the value the more useful a negative result is. The positive likelihood ratio, sensitivity/(1 - specificity), would be 0.80/0.10 = 8.

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 Biostatistics: negative likelihood 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.