Screening: length-time bias: 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 · Screening: length-time bias

Sample item

A mammography programme reports that women whose cancers were detected on screening survive longer than women whose cancers presented with a lump between screening rounds. Reviewers point out that screening preferentially detects slow-growing, less aggressive tumours, while fast-growing tumours tend to appear clinically in the intervals. Which bias is described?

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

The answer and the reasoning

B. Length-time bias

Length-time bias occurs because periodic screening is more likely to find slowly progressive disease, which has a long preclinical phase, than rapidly progressive disease. Screen-detected cases therefore seem to have better survival even if screening does not change the outcome. A randomised trial with mortality as the outcome avoids both length and lead-time bias.

Option by option

  • A. Recall bias

    Recall bias is differential recollection of exposure and does not apply to screening outcomes.

  • B. Length-time bias (correct)

    Correct. Over-representation of slow-growing disease among screen-detected cases is length-time bias.

  • C. Berkson's bias

    Berkson's bias arises from different hospital admission rates among cases and controls.

  • D. Lead-time bias

    Lead-time bias is the apparent lengthening of survival simply because diagnosis is made earlier, regardless of tumour behaviour.

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, Length-time bias. Length-time bias occurs because periodic screening is more likely to find slowly progressive disease, which has a long preclinical phase, than rapidly progressive disease.

Why is "Recall bias" not the answer?

Recall bias is differential recollection of exposure and does not apply to screening outcomes.

Why is "Berkson's bias" not the answer?

Berkson's bias arises from different hospital admission rates among cases and controls.

Why is "Lead-time bias" not the answer?

Lead-time bias is the apparent lengthening of survival simply because diagnosis is made earlier, regardless of tumour behaviour.

What concept does this question test?

It tests Screening: length-time bias in PSM. Screen-detected cases therefore seem to have better survival even if screening does not change the outcome. A randomised trial with mortality as the outcome avoids both length and lead-time bias.

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 Screening: length-time bias?

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.