Biostatistics: standard deviation: 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 deviation

Sample item

Five children from a village school have haemoglobin values of 4, 6, 8, 10 and 12 g/dL in a small sample drawn for a pilot survey. What is the sample standard deviation?

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

The answer and the reasoning

A. 3.16 g/dL

The mean is 8. The squared deviations are 16, 4, 0, 4 and 16, which sum to 40. For a small sample the sum is divided by n minus 1, that is 4, giving a variance of 10 and a standard deviation of the square root of 10, which is 3.16.

Option by option

  • A. 3.16 g/dL (correct)

    Correct. Variance 40/4 = 10, and its square root is 3.16 g/dL.

  • B. 10.0 g/dL

    This is the variance (40/4), not the standard deviation, which is its square root.

  • C. 2.83 g/dL

    This divides by n (40/5 = 8) and is the population formula, not the sample formula.

  • D. 40.0 g/dL

    This is the sum of squared deviations and has not been divided or square-rooted.

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 A, 3.16 g/dL. The mean is 8.

Why is "10.0 g/dL" not the answer?

This is the variance (40/4), not the standard deviation, which is its square root.

Why is "2.83 g/dL" not the answer?

This divides by n (40/5 = 8) and is the population formula, not the sample formula.

Why is "40.0 g/dL" not the answer?

This is the sum of squared deviations and has not been divided or square-rooted.

What concept does this question test?

It tests Biostatistics: standard deviation in PSM. The squared deviations are 16, 4, 0, 4 and 16, which sum to 40. For a small sample the sum is divided by n minus 1, that is 4, giving a variance of 10 and a standard deviation of the square root of 10, which is 3.16.

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 deviation?

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.