Epidemiology: cluster randomised trial: 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.

Try it

PSM · Epidemiology: cluster randomised trial

Sample item

In a trial of a water-chlorination programme, whole villages were randomly assigned to intervention or no intervention. Why does such a trial need a larger sample than an individually randomised trial?

AA placebo cannot be used in community trialsBIndividuals within a village are correlated, so the design effect inflates the sample sizeCVillages cannot be matched before randomisationDIntention-to-treat analysis cannot be done in cluster trials

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

The answer and the reasoning

B. Individuals within a village are correlated, so the design effect inflates the sample size

In a cluster randomised trial, people in the same cluster resemble each other, so the effective information is less than the number of participants. The design effect multiplies the sample size needed. Analysis must account for clustering.

Option by option

A. A placebo cannot be used in community trials
The lack of a placebo does not determine sample size.
B. Individuals within a village are correlated, so the design effect inflates the sample size (correct)
Correct. Intracluster correlation requires inflating the sample size by the design effect.
C. Villages cannot be matched before randomisation
Villages can be matched or stratified before randomisation.
D. Intention-to-treat analysis cannot be done in cluster trials
Intention-to-treat analysis is possible in cluster trials.

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, Individuals within a village are correlated, so the design effect inflates the sample size. In a cluster randomised trial, people in the same cluster resemble each other, so the effective information is less than the number of participants.

Why is "A placebo cannot be used in community trials" not the answer?

The lack of a placebo does not determine sample size.

Why is "Villages cannot be matched before randomisation" not the answer?

Villages can be matched or stratified before randomisation.

Why is "Intention-to-treat analysis cannot be done in cluster trials" not the answer?

Intention-to-treat analysis is possible in cluster trials.

What concept does this question test?

It tests Epidemiology: cluster randomised trial in PSM. The design effect multiplies the sample size needed. Analysis must account for clustering.

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: cluster randomised trial?

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.