Biostatistics: rank correlation: 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: rank correlation

Sample item

A study relates severity of dyspnoea graded as mild, moderate or severe to the grade of airflow obstruction on spirometry, also graded in ordered categories. Which measure of association is most suitable?

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

The answer and the reasoning

C. Spearman rank correlation coefficient

Spearman correlation uses ranks and is suited to ordinal data or to quantitative data that are not normally distributed. Pearson correlation assumes interval-scale data with a linear relation and approximately normal distribution. Both give a coefficient between minus 1 and plus 1.

Option by option

  • A. Pearson product-moment correlation coefficient

    Pearson correlation needs interval-scale, approximately normal data, not ordered grades.

  • B. Unpaired t-test

    An unpaired t-test compares means between two groups and gives no correlation.

  • C. Spearman rank correlation coefficient (correct)

    Correct. Ordinal grades are ranked, so Spearman correlation fits.

  • D. Odds ratio

    An odds ratio summarises association in a 2 x 2 table of binary variables.

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, Spearman rank correlation coefficient. Spearman correlation uses ranks and is suited to ordinal data or to quantitative data that are not normally distributed.

Why is "Pearson product-moment correlation coefficient" not the answer?

Pearson correlation needs interval-scale, approximately normal data, not ordered grades.

Why is "Unpaired t-test" not the answer?

An unpaired t-test compares means between two groups and gives no correlation.

Why is "Odds ratio" not the answer?

An odds ratio summarises association in a 2 x 2 table of binary variables.

What concept does this question test?

It tests Biostatistics: rank correlation in PSM. Pearson correlation assumes interval-scale data with a linear relation and approximately normal distribution. Both give a coefficient between minus 1 and plus 1.

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: rank correlation?

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.