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

Statistical Methods for Rates and Proportions (Wiley Series in Probability and Statistics) Statistical Methods for Rates and Proportions (Wiley Series in Probability and Statistics)
by Joseph L. Fleiss, Bruce Levin, Myunghee Cho Paik
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Prism

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