Biostatistics: combined 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: combined mean

Sample item

In a clinic, 20 women have a mean haemoglobin of 10 g/dL and 30 women from a second clinic have a mean of 11 g/dL. What is the mean haemoglobin of all 50 women?

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

The answer and the reasoning

B. 10.6 g/dL

The combined mean is a weighted mean, the total of the values divided by the total number of subjects. That is (20 x 10 + 30 x 11) divided by 50, which gives 530 divided by 50, or 10.6 g/dL. The simple average of the two means ignores the unequal group sizes.

Option by option

  • A. 11.0 g/dL

    This is the mean of the larger group alone.

  • B. 10.6 g/dL (correct)

    Correct. The weighted mean of 20 and 30 women is 10.6 g/dL.

  • C. 10.5 g/dL

    This is the simple average of 10 and 11, which ignores group sizes.

  • D. 10.4 g/dL

    This weights the group means the wrong way round.

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, 10.6 g/dL. The combined mean is a weighted mean, the total of the values divided by the total number of subjects.

Why is "11.0 g/dL" not the answer?

This is the mean of the larger group alone.

Why is "10.5 g/dL" not the answer?

This is the simple average of 10 and 11, which ignores group sizes.

Why is "10.4 g/dL" not the answer?

This weights the group means the wrong way round.

What concept does this question test?

It tests Biostatistics: combined mean in PSM. That is (20 x 10 + 30 x 11) divided by 50, which gives 530 divided by 50, or 10.6 g/dL. The simple average of the two means ignores the unequal group sizes.

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: combined 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.