Enter your data for Power and Sample Size for 2 Proportions

Stat > Power and Sample Size > 2 Proportions

Complete the following steps to specify the data for the power and sample size calculation.

  1. Specify values for two of the following power function variables. Leave the variable that you want to calculate blank.
    • Sample sizes: Enter the number of observations for each group. For example, enter 50 to indicate that you will collect 50 observations for each of the two groups. To assess the effect of different sample sizes, enter multiple values. Larger sample sizes give the test more power to detect a difference.

      If you enter multiple values into a field, separate the values with a space. You can also use shorthand notation to indicate multiple values. For example, you can enter 10:40/5 to indicate sample sizes from 10 to 40 in increments of 5.

    • Comparison proportions (p1): Enter one or more proportions that are meaningfully different from the Baseline proportion (p2). If you conduct a before/after analysis, enter the after proportion here. Usually, the difference between the comparison proportion and baseline proportion is the smallest difference that has practical consequences for your application.

    • Power values: Enter one or more values to specify the probability that the test detects a difference between the proportions when a difference actually exists. Common values are 0.8 and 0.9. For example, analysts enter 0.9 to have a 90% chance that the test will detect an important difference between the proportion of on-time deliveries before and after a process improvement when a difference actually exists.
  2. In Baseline proportion (p2), enter the proportion to which you want to compare the Comparison proportions (p1). If you conduct a before/after analysis, enter the before proportion here.