Example of Power and Sample Size for 2-Sample t

A healthcare consultant wants to compare the patient satisfaction ratings of two hospitals. Before collecting the data for a 2-sample t-test, the consultant uses a power and sample size calculation to determine the sample size required to detect a difference of 5 with a probability as high as 90% (power of 0.9). Previous studies indicate the ratings have a standard deviation of 10.

  1. Choose Stat > Power and Sample Size > 2-Sample t.
  2. In Differences, enter 5.
  3. In Power values, enter 0.9.
  4. In Standard deviation, enter 10.
  5. Click OK.

Interpret the results

To detect a difference of 5 with a power of 0.9, the consultant needs to collect a minimum sample size of 86. Because the target power value of 0.9 results in a sample size that is not an integer, Minitab also displays the power (Actual Power) for the rounded sample size.

Power and Sample Size

2-Sample t Test Testing mean 1 = mean 2 (versus ≠) Calculating power for mean 1 = mean 2 + difference α = 0.05 Assumed standard deviation = 10
Results Sample Target Difference Size Power Actual Power 5 86 0.9 0.903230 The sample size is for each group.

Power Curve for 2-Sample t Test