Example of the Heterogeneity of Variance Test
Use the Variability Chart platform to test if the variances of different groups are statistically different. In this example, three operators each took three measurements of ten parts, for a total of 90 observations. You want to examine the following:
• whether the variance of measurements for each operator are the same or different
• whether the variance for each part is the same or different
• whether the variance for each Operator*part combination is the same or different
1. Select Help > Sample Data Folder and open Variability Data/2 Factors Crossed.jmp.
2. Select Analyze > Quality and Process > Variability / Attribute Gauge Chart.
3. Select Measurement and click Y, Response.
4. Select Operator and click X, Grouping.
5. Select part# and click Part, Sample ID.
6. (Optional) Enter 1234 next to Set Random Seed.
7. Click OK.
8. Click the Variability Gauge Analysis for Measurement red triangle and select Heterogeneity of Variance Tests.
9. Select Crossed.
10. Click OK.
Figure 6.10 Heterogeneity of Variances Tests Report
In the Operator Variance test, all three levels exceed the upper and lower decision limits. From this, you conclude that each operator has a different variability from the square root of the average group variance. You might want to examine why the variation between each operator is different.
For the part# Variance test and the interaction (Operator*part#) Variance test, none of the levels exceed the decision limits. From this, you conclude that the variances are not statistically different from the square root of the average group variance. Each part has a similar variance to the other parts, and each Operator*part# combination has similar variance to the other Operator*part# combinations.