Biostatistics: standard error of a proportion: 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: standard error of a proportion

Sample item

In a sample of 100 children the prevalence of stunting is 20%. What is the standard error of this proportion?

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

The answer and the reasoning

D. 4%

The standard error of a proportion is the square root of (p x q / n). With p = 20 and q = 80 it is the square root of (20 x 80 / 100) = the square root of 16 = 4%. A 95% confidence interval would therefore be about 20 plus or minus 7.8%.

Option by option

  • A. 40%

    This is the square root of pq (40) without dividing by n.

  • B. 16%

    This is the variance term (pq/n = 16) without taking the square root.

  • C. 0.4%

    This divides the square root of pq by n rather than by the square root of n.

  • D. 4% (correct)

    Correct. The square root of (20 x 80 / 100) is 4%.

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 D, 4%. The standard error of a proportion is the square root of (p x q / n).

Why is "40%" not the answer?

This is the square root of pq (40) without dividing by n.

Why is "16%" not the answer?

This is the variance term (pq/n = 16) without taking the square root.

Why is "0.4%" not the answer?

This divides the square root of pq by n rather than by the square root of n.

What concept does this question test?

It tests Biostatistics: standard error of a proportion in PSM. With p = 20 and q = 80 it is the square root of (20 x 80 / 100) = the square root of 16 = 4%. A 95% confidence interval would therefore be about 20 plus or minus 7.8%.

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: standard error of a proportion?

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.