Biostatistics: confidence interval for a mean: 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: confidence interval for a mean

Sample item

In a survey of 100 adults the mean systolic blood pressure is 130 mmHg with a standard deviation of 20 mmHg. Which is the approximate 95% confidence interval for the population mean?

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

The answer and the reasoning

B. 126.1 to 133.9 mmHg

The standard error is SD divided by the square root of n, which is 20/10 = 2. The 95% confidence interval is mean plus or minus 1.96 times the standard error, 130 plus or minus 3.92, giving 126.1 to 133.9. It describes the precision of the estimated mean, not the spread of individual values.

Option by option

  • A. 128.0 to 132.0 mmHg

    This is the mean plus or minus one standard error, which is only a 68% interval.

  • B. 126.1 to 133.9 mmHg (correct)

    Correct. 130 plus or minus 1.96 x 2 gives 126.1 to 133.9.

  • C. 129.6 to 130.4 mmHg

    This wrongly divides the SD by n (20/100 = 0.2) rather than by the square root of n.

  • D. 90.8 to 169.2 mmHg

    This uses the standard deviation instead of the standard error and describes where individual values lie.

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 B, 126.1 to 133.9 mmHg. The standard error is SD divided by the square root of n, which is 20/10 = 2.

Why is "128.0 to 132.0 mmHg" not the answer?

This is the mean plus or minus one standard error, which is only a 68% interval.

Why is "129.6 to 130.4 mmHg" not the answer?

This wrongly divides the SD by n (20/100 = 0.2) rather than by the square root of n.

Why is "90.8 to 169.2 mmHg" not the answer?

This uses the standard deviation instead of the standard error and describes where individual values lie.

What concept does this question test?

It tests Biostatistics: confidence interval for a mean in PSM. The 95% confidence interval is mean plus or minus 1.96 times the standard error, 130 plus or minus 3.92, giving 126.1 to 133.9. It describes the precision of the estimated mean, not the spread of individual values.

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: confidence interval for a mean?

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.