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 & Proportions|
|by Joseph L. Fleiss, Bruce Levin, Myunghee Cho Paik|
|IBSN:0471526290. List price:$170.00|
|Buy from amazon.com for $115.73|