Biostatistics: reference interval: 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: reference interval

Sample item

A laboratory defines the reference range for a test as the mean plus or minus 2 standard deviations of healthy people. A physician notes that about 1 in 20 perfectly healthy persons is labelled abnormal. Why does this happen?

AThe healthy sample was not drawn at randomBThe test has a sensitivity of only 95%CThe standard deviation has been calculated incorrectlyDBy construction, about 5% of healthy people lie outside the range

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

The answer and the reasoning

D. By construction, about 5% of healthy people lie outside the range

In a normal distribution, mean plus or minus 2 SD includes about 95% of values, so about 5% of healthy persons are expected to fall outside by definition. This is why borderline results in a healthy person need not mean disease. The range describes the distribution of healthy values, not test accuracy.

Option by option

A. The healthy sample was not drawn at random
Non-random sampling would distort the range, but the 5% result is inherent in the method.
B. The test has a sensitivity of only 95%
Sensitivity concerns detection of diseased persons, not the reference interval in healthy people.
C. The standard deviation has been calculated incorrectly
No calculation error is needed to explain the expected 5%.
D. By construction, about 5% of healthy people lie outside the range (correct)
Correct. A 95% interval necessarily leaves about 5% of healthy people outside.

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 D, By construction, about 5% of healthy people lie outside the range. In a normal distribution, mean plus or minus 2 SD includes about 95% of values, so about 5% of healthy persons are expected to fall outside by definition.

Why is "The healthy sample was not drawn at random" not the answer?

Non-random sampling would distort the range, but the 5% result is inherent in the method.

Why is "The test has a sensitivity of only 95%" not the answer?

Sensitivity concerns detection of diseased persons, not the reference interval in healthy people.

Why is "The standard deviation has been calculated incorrectly" not the answer?

No calculation error is needed to explain the expected 5%.

What concept does this question test?

It tests Biostatistics: reference interval in PSM. This is why borderline results in a healthy person need not mean disease. The range describes the distribution of healthy values, not test accuracy.

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: reference interval?

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.