Overview for Power and Sample Size for 2 Variances

Use Power and Sample Size for 2 Variances to examine the relationship between power, sample size, and the ratio when you want to compare the ratio between two population variances or standard deviations to a target or reference value. Use these calculations for the following reasons:
  • Before you collect data for a 2 variances test, to ensure that your test has an adequate sample size to achieve acceptable power
  • After a 2 variances test, to improve the design for the next test

For example, managers of a car manufacturer want to compare the variances for the diameters of camshafts for two different vendors. Before collecting the data for a 2 variances test, the manufacturer uses a power and sample size calculation to determine the smallest ratio a test with a specific power value for a given sample size can detect.

This test is useful for quality improvement situations: you can use this test to compare variance within subgroups to variance between subgroups. Also, you can use this test to compare process variance before and after implementation of a quality improvement program.

Where to find this analysis

To perform a power and sample size calculation for a 2 variances test, choose Stat > Power and Sample Size > 2 Variances.

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