Biostatistics: normal distribution tail: 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: normal distribution tail

Sample item

Serum cholesterol in a population is normally distributed with a mean of 200 mg/dL and a standard deviation of 20 mg/dL. Approximately what percentage of people have values above 240 mg/dL?

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

The answer and the reasoning

C. 2.5%

240 mg/dL is two standard deviations above the mean. About 95% of values lie within two standard deviations, so about 5% lie outside, and half of that, 2.5%, lies above. About 16% lie above one standard deviation and about 0.15% above three.

Option by option

  • A. 5%

    5% is the total outside both tails, not only the upper one.

  • B. 0.15%

    0.15% is the proportion above mean plus three SD.

  • C. 2.5% (correct)

    Correct. Values beyond mean plus two SD make up about 2.5% in the upper tail.

  • D. 16%

    16% is the proportion above mean plus one SD.

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 C, 2.5%. 240 mg/dL is two standard deviations above the mean.

Why is "5%" not the answer?

5% is the total outside both tails, not only the upper one.

Why is "0.15%" not the answer?

0.15% is the proportion above mean plus three SD.

Why is "16%" not the answer?

16% is the proportion above mean plus one SD.

What concept does this question test?

It tests Biostatistics: normal distribution tail in PSM. About 95% of values lie within two standard deviations, so about 5% lie outside, and half of that, 2.5%, lies above. About 16% lie above one standard deviation and about 0.15% above three.

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 Biostatistics: normal distribution tail?

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.