Enter your data for Power and Sample Size for 2-Sample t

Stat > Power and Sample Size > 2-Sample t

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.

    • Differences: Enter one or more values to specify the difference in means that you want to detect between two groups. Usually, you enter the smallest difference that has practical consequences for your application.

      If you select Less than on the Options sub-dialog box, enter a negative difference. Otherwise, enter a positive difference.

    • Power values: Enter one or more values to specify the probability that the test detects the difference between the means when a difference actually exists. Common values are 0.8 and 0.9. For example, analysts enter 0.9 because they want a 90% chance that the test will detect an important difference between two assembly lines when a difference actually exists.
  2. In Standard deviation, enter a planning estimate of the common standard deviation of the two populations (denoted as σ or sigma). Base your estimate on related research, design specifications, pilot studies, subject-matter knowledge, or similar information. If you have already collected and analyzed the data, use the pooled standard deviation of the samples.