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9B: Comparing two proportions

9B: Comparing two proportions. Proportions from two groups.

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9B: Comparing two proportions

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  1. 9B: Comparing two proportions 9B: Comparing Proportions

  2. Proportions from two groups A trial compares heart attack occurrences over 5-years in two groups of middle-aged men. Group 1 receives a placebo while group 2 receives the cholesterol-lowering drug Gemfibrozil. We want to determine whether the drug reduces heart attack risk. The risk difference is the difference (subtraction) in proportions: 9B: Comparing Proportions

  3. CI for risk difference (+4 methods) Goal: Estimate parameter p1 – p2 “Plus four” method adds one pretend success and failure to each group: Example: same data 95% CI for p1 – p2 is given by: Interpretation references parameter: We are 95% confidence the risk difference in the population is between 0.3% and 2.5%. 9B: Comparing Proportions

  4. Significance test Example: same data A. H0: p1 = p2 (no difference in population proportions) B. Test statistics C. P = 0.01351 D. The evidence against the null is significant. . 9B: Comparing Proportions

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