Biostatistics: sample size and precision: 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: sample size and precision

Sample item

A planner calculates the sample size needed to estimate a prevalence with an absolute precision of 5 percent. What happens to the required sample size if the precision demanded is tightened to 2.5 percent?

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

The answer and the reasoning

C. It becomes four times larger

Sample size varies inversely with the square of the allowable error, so halving the allowable error multiplies the sample size by four. This is why very precise estimates are costly. Variability and confidence level also affect it.

Option by option

  • A. It halves

    Tightening precision cannot reduce the sample size required.

  • B. It remains unchanged

    Precision is a direct determinant of sample size, so it cannot remain unchanged.

  • C. It becomes four times larger (correct)

    Correct. Sample size is proportional to 1 divided by the square of the allowable error.

  • D. It doubles

    Doubling would apply if the relationship were linear, which it is not.

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 C, It becomes four times larger. Sample size varies inversely with the square of the allowable error, so halving the allowable error multiplies the sample size by four.

Why is "It halves" not the answer?

Tightening precision cannot reduce the sample size required.

Why is "It remains unchanged" not the answer?

Precision is a direct determinant of sample size, so it cannot remain unchanged.

Why is "It doubles" not the answer?

Doubling would apply if the relationship were linear, which it is not.

What concept does this question test?

It tests Biostatistics: sample size and precision in PSM. This is why very precise estimates are costly. Variability and confidence level also affect it.

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: sample size and precision?

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.