Biostatistics: width of the confidence 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: width of the confidence interval

Sample item

Two district surveys both report an anaemia prevalence of 40 percent. The 95% confidence interval is 35 to 45 percent in District X and 38 to 42 percent in District Y. If the methods are otherwise identical, what is the most likely reason for the narrower interval in District Y?

AUse of a 99% confidence levelBA larger sample sizeCA smaller sample sizeDA lower true prevalence

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

The answer and the reasoning

B. A larger sample size

The width of a confidence interval depends on the standard error, which falls as sample size rises. A larger sample therefore gives a narrower interval, while a higher confidence level makes it wider.

Option by option

A. Use of a 99% confidence level
Both intervals are stated at 95 percent, and a 99 percent level would widen an interval.
B. A larger sample size (correct)
Correct. A larger sample reduces the standard error and so narrows the interval.
C. A smaller sample size
A smaller sample would widen the interval, not narrow it.
D. A lower true prevalence
Both districts have the same observed prevalence, so a different true prevalence does not explain the difference in width.

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 B, A larger sample size. The width of a confidence interval depends on the standard error, which falls as sample size rises.

Why is "Use of a 99% confidence level" not the answer?

Both intervals are stated at 95 percent, and a 99 percent level would widen an interval.

Why is "A smaller sample size" not the answer?

A smaller sample would widen the interval, not narrow it.

Why is "A lower true prevalence" not the answer?

Both districts have the same observed prevalence, so a different true prevalence does not explain the difference in width.

What concept does this question test?

It tests Biostatistics: width of the confidence interval in PSM. A larger sample therefore gives a narrower interval, while a higher confidence level makes it wider.

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: width of the confidence 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.