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. Open the Test for Equal Variances dialog box.
    • Mac: Statistics > ANOVA > Equal Variances
    • PC: STATISTICS > ANOVA > Equal Variances
  3. Select Responses are in one column for all factor levels.
  4. In Response, enter Correction Time.
  5. In Factors, enter Experience RoadType.
  6. Click OK.

Interpret the results

In the output, both of the p-values are greater than the common significance level of 0.05. On the plot with the multiple comparison intervals, all the comparison intervals overlap. The p-values and the comparison intervals both indicate that the differences between the groups are not statistically significant.

95% Bonferroni Confidence Intervals for Standard Deviations
Individual confidence level = 99.1667%
Null hypothesis
H₀: All variances are equal
Alternative hypothesis
H₁: At least one variance is different
Test Statistic
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