I am exploring power/sample size and confidence intervals for differences in proportions (paired samples) using the software that comes with Altman et al (2000) CIA and DSTPlan.
Changing the proportions in CIA with a sample sizes of 30 and 70 suggests that the Confidence level approach to determining Statistical significance is very sensitive ie CI range not including zero. Even with the smaller sample size a change in proportions of .1 is considered statistically significant at the 0.1 level. I was in fact surprised at how sensitive this was and played around with several proportions and effect sizes just to confirm these findings However, using DSTPlan with the routine for detecting the difference between two proportions with permanent sampling units suggests that my sample size of 70 has a power of only 55% to detect a 10% change at the 0.1 significance level and the smaller sample has a power of only 17%. The minimum sample size to detect my 10% change at a significance level of 0.1 is 134. These results seem some what contradictory, and obviously there is something I am not grasping in what I am doing. Can anyone make any suggestions as to what I am doing wrong and possibly point me towards some reading or web sites. Many thanks, Graham S . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
