Tests of Differences between Proportions (2 of 5)

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  1. The second step is to choose a significance level. Assume the 0.05 level is chosen.

  2. The third step is to compute the difference between the sample proportions. In this example, p1 - p2 = 73/85 - 43/82 = 0.8588 - 0.5244 = 0.3344.

  3. The fourth step is to compute p, the probability (or probability value). It is the probability of obtaining a difference between the proportions as large or larger than the difference observed in the experiment. Applying the general formula to the problem of differences between proportions

    where p1- p2 is the difference between sample proportions and

    is the estimated standard error of the difference between proportions. The formula for the estimated standard error is:

    where p is a weighted average of the p1 and p2, n1 is the number of subjects sampled from the first population, and n2 is the number of subjects sampled from the second population.
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