1. Topic 21—Comparison of Proportions Example

2. Test of Significance A test of significance for the difference of two proportions Requirements: – Indep. SRS or Random assign. to groups? Randomly assigned to groups—OK –. – Results should be valid

3. Ho: Pa = Pp The proportion of all HIV+ people who develop AIDS on AZT equals proportion of HIV+ people who develop AIDS on placebo Ha: Pa < Pp The proportion of all HIV+ people who develop AIDS on AZT is less than the proportion of HIV+ people who develop AIDS on placebo Z = P(z<-2.9256) =.0017

4. Because our p-value is less than 5%, we reject the null hypothesis. We have enough evidence to conclude that the proportion of HIV+ people who develop AIDS on AZT is less than the proportion of HIV+ people who develop AIDS on a placebo.

5. Confidence Interval 95% confidence interval to estimate the difference between 2 proportions Requirements – Same as above..

6. (-.0805, -.0161) I am 95% confident that the true difference between the proportions who developed AIDS between the AZT and placebo groups is between -.0805 and -.0161 ***AZT group developed AIDS between 1.6% and 8% less often.