Biostatistics: degrees of freedom in chi-square 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: degrees of freedom in chi-square test

Sample item

A researcher tests the association between three categories of body mass index and the presence or absence of hypertension, using a chi-square test on a table with 3 rows and 2 columns. How many degrees of freedom does the test have?

A3B2C6D5

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

The answer and the reasoning

B. 2

For a contingency table the degrees of freedom are (rows minus 1) multiplied by (columns minus 1). Here that is (3 - 1) x (2 - 1) = 2. For a 2 x 2 table the answer is 1.

Option by option

A. 3
Three is the number of rows, and one is not subtracted.
B. 2 (correct)
Correct. (3 - 1) x (2 - 1) gives 2.
C. 6
Six is the number of cells in the table, not the degrees of freedom.
D. 5
Five comes from adding rows and columns, which is not how degrees of freedom are calculated.

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, 2. For a contingency table the degrees of freedom are (rows minus 1) multiplied by (columns minus 1).

Why is "3" not the answer?

Three is the number of rows, and one is not subtracted.

Why is "6" not the answer?

Six is the number of cells in the table, not the degrees of freedom.

Why is "5" not the answer?

Five comes from adding rows and columns, which is not how degrees of freedom are calculated.

What concept does this question test?

It tests Biostatistics: degrees of freedom in chi-square test in PSM. Here that is (3 - 1) x (2 - 1) = 2. For a 2 x 2 table the answer is 1.

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: degrees of freedom in chi-square 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.