Epidemiology: specificity and false positives: 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: specificity and false positives

Sample item

A screening test has a specificity of 95 percent. When it is used on 2,000 persons who are truly free of the disease, how many are expected to test positive?

A100B50C95D1,900

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

The answer and the reasoning

A. 100

Specificity is the proportion of disease-free people who test negative. At 95 percent, 1,900 test negative and 5 percent, or 100 of 2,000, are false positives. The false positive rate is 1 minus specificity.

Option by option

A. 100 (correct)
Correct. Five percent of 2,000 is 100 false positives.
B. 50
Fifty would be 2.5 percent, which corresponds to 97.5 percent specificity.
C. 95
Ninety-five is the percentage specificity, not a count.
D. 1,900
One thousand nine hundred is the number of true negatives.

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 A, 100. Specificity is the proportion of disease-free people who test negative.

Why is "50" not the answer?

Fifty would be 2.5 percent, which corresponds to 97.5 percent specificity.

Why is "95" not the answer?

Ninety-five is the percentage specificity, not a count.

Why is "1,900" not the answer?

One thousand nine hundred is the number of true negatives.

What concept does this question test?

It tests Epidemiology: specificity and false positives in PSM. At 95 percent, 1,900 test negative and 5 percent, or 100 of 2,000, are false positives. The false positive rate is 1 minus specificity.

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: specificity and false positives?

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.