Biostatistics: limits of the correlation coefficient: 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: limits of the correlation coefficient

Sample item

In a hospital series, birth weight is plotted against maternal age. Very young and older mothers have low-weight babies, while mothers in the middle age range have heavier babies. The Pearson correlation coefficient for the data is about 0.04. What is the best interpretation?

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

The answer and the reasoning

D. It captures only straight-line association, so a curved relationship can still be present

Pearson r measures the strength of a linear relationship. A U-shaped or inverted U-shaped relationship can be strong yet yield an r near zero, so a scatter diagram should always be inspected, and r near zero does not prove independence.

Option by option

  • A. Maternal age and birth weight are unrelated

    The scatter pattern shows a clear relationship, so the variables are not unrelated.

  • B. The coefficient is invalid because birth weight is continuous

    Pearson r is designed for continuous variables, so continuity of birth weight does not invalidate it.

  • C. A weak positive relationship exists, so older mothers have heavier babies

    A value of 0.04 is essentially zero and does not show that older mothers have heavier babies.

  • D. It captures only straight-line association, so a curved relationship can still be present (correct)

    Correct. A strong curved relationship can coexist with an r near zero because r measures only linear trend.

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 D, It captures only straight-line association, so a curved relationship can still be present. Pearson r measures the strength of a linear relationship.

Why is "Maternal age and birth weight are unrelated" not the answer?

The scatter pattern shows a clear relationship, so the variables are not unrelated.

Why is "The coefficient is invalid because birth weight is continuous" not the answer?

Pearson r is designed for continuous variables, so continuity of birth weight does not invalidate it.

Why is "A weak positive relationship exists, so older mothers have heavier ba…" not the answer?

A value of 0.04 is essentially zero and does not show that older mothers have heavier babies.

What concept does this question test?

It tests Biostatistics: limits of the correlation coefficient in PSM. A U-shaped or inverted U-shaped relationship can be strong yet yield an r near zero, so a scatter diagram should always be inspected, and r near zero does not prove independence.

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: limits of the correlation coefficient?

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.