Epidemiology: block randomisation: 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: block randomisation

Sample item

A trial allocates every consecutive set of four enrolees so that exactly two receive each treatment, with the order within the set decided at random. What is the main purpose of this method?

ATo remove the need for blinding of the investigatorBTo keep the two groups nearly equal in size throughout recruitmentCTo allow staff to predict the next allocationDTo make unknown confounders identical in the two groups

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

The answer and the reasoning

B. To keep the two groups nearly equal in size throughout recruitment

Block randomisation keeps group sizes balanced at the end of every block, which matters if recruitment stops early. It does not guarantee balance of confounders, which depends on sample size. It also does not replace blinding or concealment, and fixed small blocks can in fact be guessed if not concealed.

Option by option

A. To remove the need for blinding of the investigator
Blinding is a separate safeguard against observer bias and is not removed by this method.
B. To keep the two groups nearly equal in size throughout recruitment (correct)
Correct. Each block of four contains two of each arm, so sizes stay balanced.
C. To allow staff to predict the next allocation
Predictability is a drawback to avoid, not a purpose.
D. To make unknown confounders identical in the two groups
Balance of unknown confounders is expected from random allocation in large samples, not guaranteed by blocks.

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, To keep the two groups nearly equal in size throughout recruitment. Block randomisation keeps group sizes balanced at the end of every block, which matters if recruitment stops early.

Why is "To remove the need for blinding of the investigator" not the answer?

Blinding is a separate safeguard against observer bias and is not removed by this method.

Why is "To allow staff to predict the next allocation" not the answer?

Predictability is a drawback to avoid, not a purpose.

Why is "To make unknown confounders identical in the two groups" not the answer?

Balance of unknown confounders is expected from random allocation in large samples, not guaranteed by blocks.

What concept does this question test?

It tests Epidemiology: block randomisation in PSM. It does not guarantee balance of confounders, which depends on sample size. It also does not replace blinding or concealment, and fixed small blocks can in fact be guessed if not concealed.

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: block randomisation?

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.