Using the Power and Sample Size tool for a 2 Proportions test

In the previous recipe, we checked if we could prove a difference in the population of smokers between a group of students. Now, we will check how many students we need to sample to observe a difference of 5 percent or 10 percent between smokers of each gender.

Getting ready

We will use the figures from the previous recipe, Testing two population proportions with the 2 Proportions test, but there is no need to open a dataset.

How to do it…

The following steps will find the number of male and female students that need to be included in a sample to be able to observe a difference in the proportion of smokers in each group of 0.05 or 0.1 with at least 80 percent or 90 percent power:

  1. Go to the Stat menu, select Power and Sample Size, and click on 2 Proportions….
  2. Enter 0.25 in the Baseline proportion (p2): field; in Comparison proportions (p1):, enter 0.15 0.2 0.3 0.35.
  3. In Power values:, enter .8 .9.
  4. Click on OK.

How it works…

The sample size that is calculated is for each group, not overall. To have an 80 percent chance of seeing a population difference of 0.25 to 0.35 in the proportion of smokers, we would need 329 male and female students.

See also

  • The Testing two population proportions with the 2 Proportions test recipe
  • The Using the Assistant menu to run a 2 Proportions test recipe
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