Degree of freedom of a chi square test in contingency table of 2 by 2 is?

**Core Concept**
A chi-square test is a statistical method used to determine if there is a significant association between two categorical variables. In a 2x2 contingency table, the chi-square test is used to evaluate the relationship between two binary variables. The degree of freedom (df) is a critical component of the chi-square test, representing the number of values in the final estimate of a parameter that are free to vary.

**Why the Correct Answer is Right**
The degree of freedom for a 2x2 contingency table is calculated as (r-1) x (c-1), where r is the number of rows and c is the number of columns. For a 2x2 table, r=2 and c=2, so the degree of freedom is (2-1) x (2-1) = 1 x 1 = 1. This means that there is only one value in the table that is free to vary.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately calculate the degree of freedom for a 2x2 contingency table.
* **Option B:** This option is incorrect because it represents the number of rows (r) and not the degree of freedom.
* **Option C:** This option is incorrect because it represents the number of columns (c) and not the degree of freedom.

**Clinical Pearl / High-Yield Fact**
Recall that the degree of freedom for a chi-square test is calculated as (r-1) x (c-1), where r is the number of rows and c is the number of columns. This formula is essential for determining the correct degree of freedom for various types of contingency tables.

**Correct Answer: C. 1**