Screening: predictive value: 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 · Screening: predictive value

Sample item

A screening test is applied to 100 people with a disease and 200 without. It is positive in 80 of the diseased and in 40 of the non-diseased. What is the positive predictive value?

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

The answer and the reasoning

D. About 67%

Positive predictive value is true positives divided by all test positives: 80 / (80 + 40) = 66.7%. Sensitivity is 80/100 = 80% and specificity is 160/200 = 80%. PPV falls as prevalence falls, so screening in low-prevalence populations yields many false positives.

Option by option

  • A. About 89%

    This is the negative predictive value, 160 ÷ 180.

  • B. 50%

    This does not follow from the table; 80 true positives among 120 test positives is not half.

  • C. 80%

    This is the sensitivity (80 of 100 diseased), not the PPV.

  • D. About 67% (correct)

    Correct. 80 ÷ 120 ≈ 67%.

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, About 67%. Positive predictive value is true positives divided by all test positives: 80 / (80 + 40) = 66.

Why is "About 89%" not the answer?

This is the negative predictive value, 160 ÷ 180.

Why is "50%" not the answer?

This does not follow from the table; 80 true positives among 120 test positives is not half.

Why is "80%" not the answer?

This is the sensitivity (80 of 100 diseased), not the PPV.

What concept does this question test?

It tests Screening: predictive value in PSM. 7%. Sensitivity is 80/100 = 80% and specificity is 160/200 = 80%. PPV falls as prevalence falls, so screening in low-prevalence populations yields many false positives.

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 Screening: predictive value?

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.