ANOVA --checking assumptions
 

• To Check the Normality of the Residuals
• To Check for Constant Variance

To Check for Constant Variance

1. To Check for Constant Variance you are going to want to create boxplots of each of the levels for each of the factors.
2. To do this go to Graph, Boxplot. The following screen should appear.

1. Enter the response varaible in for y and enter the factors in the column for X. You should get three sets of boxplots.

 

To Check the Normality of the Residuals.

1. Enter the values into one column and then list the levels of the factorA  in the second column, the levels of factor B in the third column and the levels of factor C in the third column . Your worksheet should look like this.

 length       Time        Heat      Machine
   6              8             W             A
   9              8             W             A
   1              8             W             A
   3              8             W             A
   7              8             W             B

1. Go to Stat, Basic Statistics, ANOVA, Balanced ANOVA. The following screen should appear.

1. Enter the values into the Response box.
2. Under the Model box, you need to enter the names of the three factors that you are using.
3. You also need to add all of the interaction terms. If you are do a three factor problem, you should have four itneractions. For this problem, there is time*heat, heat*machine, time*machine, and the three way interaction, time*machine*heat.
4. Now, Click on Storage. The following box should appear.

1. Click on Residuals.
2. Click on O.K. twice.
3. You will get the ANOVA output and an additional column will be added to your worksheet called "RESID1".
4. Now create a histogram of the errors by going to graph, histogram and selecting "RESID1" as your x.

1. Hit o.k. and a graph similar to the following should appear.