Biostatistics: Fisher exact test: 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: Fisher exact test

Sample item

In a pilot study of 20 patients, a 2 x 2 table comparing a new antiseptic against standard care for wound infection has an expected frequency of 3 in one cell. Which test of association should be used?

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

The answer and the reasoning

A. Fisher exact test

The chi-square test is an approximation that is reliable only when all expected cell counts are at least 5. When a 2 x 2 table has a small expected count, the Fisher exact test computes the probability directly and is the right choice. The z-test for proportions has the same large-sample requirement.

Option by option

  • A. Fisher exact test (correct)

    Correct. A small expected cell count calls for the exact probability method.

  • B. Chi-square test without correction

    Uncorrected chi-square is unreliable when an expected count is below 5.

  • C. Z-test for two proportions

    The z-test for proportions is also a large-sample approximation.

  • D. Paired t-test

    A paired t-test compares means of related measurements, not proportions.

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.

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Questions about this PSM question

What is the answer to this PSM question?

The best answer is A, Fisher exact test. The chi-square test is an approximation that is reliable only when all expected cell counts are at least 5.

Why is "Chi-square test without correction" not the answer?

Uncorrected chi-square is unreliable when an expected count is below 5.

Why is "Z-test for two proportions" not the answer?

The z-test for proportions is also a large-sample approximation.

Why is "Paired t-test" not the answer?

A paired t-test compares means of related measurements, not proportions.

What concept does this question test?

It tests Biostatistics: Fisher exact test in PSM. When a 2 x 2 table has a small expected count, the Fisher exact test computes the probability directly and is the right choice. The z-test for proportions has the same large-sample requirement.

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: Fisher exact test?

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.