72013-03-08 10:17:36localhostconverted to 1.6 markup62010-08-27 11:19:41PeterWatson52010-08-27 09:26:54PeterWatson42010-08-25 16:00:00PeterWatson32010-08-25 15:59:27PeterWatson22010-08-25 15:56:35PeterWatson12010-08-25 15:56:28PeterWatson

The two-sample chi-square test (e.g. CROSSTABS in SPSS) can be used to test statistical significance of the difference in two independent proportions but this test does not provide a confidence interval on the magnitude of this difference. Newcombe (1998) further goes on to compare various methods for giving confidence intervals for the difference in a pair of proportions from two different groups. he concludes that the Wald confidence interval is biased when the difference in proportions is close to zero or (+/-) one. As for a single proportion he recommends the two-sample analogue of the Wilson one-sample confidence interval. This EXCEL spreadsheet computes the two sample analogues of the methods in the one-sample spreadsheet. References Moore DS & McCabe GP (2006) Introduction to the practice of statistics (5th ed.). WH Freeman:New York. (This mentions the two sample analogue of the Agresti-Coull method which was not mentioned in the original paper which dealt with the one-sample case). Newcombe RG (1998) Interval estimation for the difference between independent proportions: comparison of eleven methods. Statistics in Medicine 17 873-890.