Biostatistics: one-tailed and two-tailed tests: 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 · Biostatistics: one-tailed and two-tailed tests

Sample item

A researcher compares antibody titres after a new vaccine and after the existing vaccine. There is no prior reason to expect the new vaccine to be better rather than worse. Which approach is appropriate?

AA one-tailed test, since the sample size is smallBA two-tailed test, since a difference in either direction mattersCA one-tailed test, since the new vaccine is expected to be betterDA one-tailed test in the unfavourable direction only

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

The answer and the reasoning

B. A two-tailed test, since a difference in either direction matters

A two-tailed test is used when a difference in either direction would be important and no direction was specified in advance. A one-tailed test is justified only when a difference in one direction is impossible or irrelevant, and the direction must be fixed before the data are seen. Sample size does not decide the choice.

Option by option

A. A one-tailed test, since the sample size is small
Small samples do not justify a one-tailed test; the direction of hypothesis does.
B. A two-tailed test, since a difference in either direction matters (correct)
Correct. The alternative hypothesis here is simply that the titres differ.
C. A one-tailed test, since the new vaccine is expected to be better
This assumes a direction that is not justified in advance and inflates the chance of a false positive.
D. A one-tailed test in the unfavourable direction only
Testing only the unfavourable direction would ignore a benefit, which is also of interest.

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, A two-tailed test, since a difference in either direction matters. A two-tailed test is used when a difference in either direction would be important and no direction was specified in advance.

Why is "A one-tailed test, since the sample size is small" not the answer?

Small samples do not justify a one-tailed test; the direction of hypothesis does.

Why is "A one-tailed test, since the new vaccine is expected to be better" not the answer?

This assumes a direction that is not justified in advance and inflates the chance of a false positive.

Why is "A one-tailed test in the unfavourable direction only" not the answer?

Testing only the unfavourable direction would ignore a benefit, which is also of interest.

What concept does this question test?

It tests Biostatistics: one-tailed and two-tailed tests in PSM. A one-tailed test is justified only when a difference in one direction is impossible or irrelevant, and the direction must be fixed before the data are seen. Sample size does not decide the choice.

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 Biostatistics: one-tailed and two-tailed tests?

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.