Epidemiology: ecological fallacy: 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 · Epidemiology: ecological fallacy

Sample item

A study of 40 countries finds a strong positive correlation between national average salt intake and national stroke mortality. A reader concludes that individuals who eat more salt are therefore at higher personal risk of fatal stroke. Which error is this?

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

The answer and the reasoning

D. Ecological fallacy, as group-level data cannot establish individual-level associations

In an ecological study the unit of analysis is a population, so an association between group averages may not hold for individuals within those groups. The finding can generate hypotheses, but confounders operating at the country level may produce the correlation. Individual-level inferences need cohort or case-control data.

Option by option

  • A. Information bias from misclassification of the outcome

    No misclassification of the outcome is described; the error is in extrapolating from groups to individuals.

  • B. Type II error due to inadequate sample size

    A strong correlation was found, so the problem is not failure to detect an association.

  • C. Selection bias due to non-random sampling of individuals

    The problem lies in the level of inference, not in how individuals were sampled.

  • D. Ecological fallacy, as group-level data cannot establish individual-level associations (correct)

    Applying a group-level correlation to individuals is the ecological fallacy.

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 D, Ecological fallacy, as group-level data cannot establish individual-level associations. In an ecological study the unit of analysis is a population, so an association between group averages may not hold for individuals within those groups.

Why is "Information bias from misclassification of the outcome" not the answer?

No misclassification of the outcome is described; the error is in extrapolating from groups to individuals.

Why is "Type II error due to inadequate sample size" not the answer?

A strong correlation was found, so the problem is not failure to detect an association.

Why is "Selection bias due to non-random sampling of individuals" not the answer?

The problem lies in the level of inference, not in how individuals were sampled.

What concept does this question test?

It tests Epidemiology: ecological fallacy in PSM. The finding can generate hypotheses, but confounders operating at the country level may produce the correlation. Individual-level inferences need cohort or case-control data.

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 Epidemiology: ecological fallacy?

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.