Example of Test for Equal Variances

A safety analyst wants to compare the variability in steering correction times for experienced and inexperienced drivers on three types of roads: paved, gravel, and dirt. The analyst records the time in seconds that each driver uses to make steering corrections on each type of road.

  1. Open the sample data, RoadConditions.MTW.
  2. Choose Stat > ANOVA > Test for Equal Variances.
  3. Select Response data are in one column for all factor levels.
  4. In Response, enter 'Correction Time'.
  5. In Factors, enter Experience and RoadType.
  6. Click OK.

Interpret the results

In this summary plot, the p-values of 0.943 and 0.830 are greater than the common significance level of 0.05. None of the differences between the groups are statistically significant, and all the comparison intervals overlap.

Test for Equal Variances: Correction Time versus Experience, RoadType

Method Null hypothesis All variances are equal Alternative hypothesis At least one variance is different Significance level α = 0.05
95% Bonferroni Confidence Intervals for Standard Deviations Experience RoadType N StDev CI Advanced Dirt 4 5.09902 (0.45310, 168.555) Advanced Gravel 4 4.16333 (0.42041, 121.108) Advanced Paved 4 3.87298 (0.31299, 140.776) Beginner Dirt 4 6.05530 (1.01104, 106.529) Beginner Gravel 4 4.76095 (0.89964, 74.009) Beginner Paved 4 5.88784 (0.52320, 194.630) Individual confidence level = 99.1667%
Tests Test Method Statistic P-Value Multiple comparisons — 0.943 Levene 0.42 0.830