Frequently Asked Questions

The z test to compare two proportions.

FAQ# 1571    Last Modified 4-February-2010

 GraphPad Prism (and InStat) offer two tests to compare two proportions: Fishers exact test and the chi-square test.

The chi-square test is equivalent to the z test. The two make the same assumptions and report the same P value. The only difference is that the chi-square test computes a value for chi-square, which Prism reports (see page 55 of Fleiss, reference below). The z test, in contrast, computes a value for the z ratio. Prism and InStat do not report this ratio, but it would equal the square root of chi-square. 

When comparing two proportions, the chi-square test can be done two ways: With and without the Yates' correction. Likewise, the z test, can be done with and without the Yates' correction.

Don't mix up this use of the z ratio (or z test), with the z factor used to quantify the quality of a screening test

Keywords: z ratio


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