Epidemiology: non-differential misclassification: 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: non-differential misclassification

Sample item

In a cohort study, exposure is measured by the same crude questionnaire in all participants, and errors occur equally among those who later do and do not develop the disease. What is the usual effect on the observed relative risk?

AIt is biased away from the null valueBIt is biased towards the null valueCIt may move in either direction unpredictablyDIt is unaffected and only the precision falls

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

The answer and the reasoning

B. It is biased towards the null value

Non-differential misclassification, where error is unrelated to the outcome, makes the groups more alike and usually pulls the measured association towards the null. Differential misclassification can bias in either direction. A real effect can therefore be hidden by a poor but unbiased measuring tool.

Option by option

A. It is biased away from the null value
Bias away from the null is not the expected result when error is unrelated to the outcome.
B. It is biased towards the null value (correct)
Correct. Equal error in both groups dilutes the contrast and moves the estimate towards the null.
C. It may move in either direction unpredictably
Unpredictable direction is typical of differential misclassification, not non-differential.
D. It is unaffected and only the precision falls
Misclassification changes the estimate itself and not merely its precision.

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.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is B, It is biased towards the null value. Non-differential misclassification, where error is unrelated to the outcome, makes the groups more alike and usually pulls the measured association towards the null.

Why is "It is biased away from the null value" not the answer?

Bias away from the null is not the expected result when error is unrelated to the outcome.

Why is "It may move in either direction unpredictably" not the answer?

Unpredictable direction is typical of differential misclassification, not non-differential.

Why is "It is unaffected and only the precision falls" not the answer?

Misclassification changes the estimate itself and not merely its precision.

What concept does this question test?

It tests Epidemiology: non-differential misclassification in PSM. Differential misclassification can bias in either direction. A real effect can therefore be hidden by a poor but unbiased measuring tool.

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 Epidemiology: non-differential misclassification?

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.