Analysis of Variance Using SAS
Analysis of Variance is the preferred statistical technique when:- The dependent variable is numeric and approximately normally distributed, AND
- The hypothesis to be tested is whether there is a significant difference among the means of three or more groups, OR
There are two or more factors to be tested, and at least one of those factors is categorical.
- Comparing the average weights of women in three different groups; vegetarian, vegan and non-vegetarian,
- Comparing scores on a Life Satisfaction Scale given to faculty and students in the schools of Engineering, Business, Arts & Science and Medicine, to test whether there are differences across schools, between faculty and staff, and whether any differences between faculty and staff are consistent across schools.
- Testing whether students receiving either a whole-language or a phonics-based curriculum improved more from pre- test to post-test.
Class categorical variable(s) ;
Model dependent = independents ;
Usually, you will also want to add a statement requesting post hoc tests. Post hoc is Latin for "after this". The phrase is meant literally. A post hoc test is necessary ONLY after a significant F-statistic is found. What Does an F-Test Test? An F-test tests whether the variance among groups is greater than the variance between groups. If your F-test is significant, showing that at least one group is significantly different from the others, the next question is, "Which group is significantly different?" This question is answered by use of a post hoc test. If the F-test is non-significant, leading to the conclusion that none of the groups are significantly different, then obviously it doesn't make any sense to do a post hoc test and see which one or more of them is different. There are several options for post hoc tests available using SAS. The most commonly used is the Tukey test. If you have three groups, A, B and C, the Tukey tests whether A is significantly different from B or C, and whether B is significantly different than C. In other words, it does all possible pair-wise comparisons. Click here for examples of how to use SAS to perform the two most commonly requested types of ANOVAs.
Last updated:
September 04, 2008