# Reduced models after finding significant interactions

If you run an ANOVA and find that there are significant interactions, do you then pull the variables out individually and re-run the ANOVA?

I am using Type III. I want to see if there is a difference in survival between 3 groups, but I have to include 3 variables other than group in my model to make sure they didn't influence survival as well. When I do that A affects survival, and so do A*D and A*E, but when I run A, D, and E by themselves, nothing affects survival.

I wish had a better grasp on stats, because I am still so confused. In essence, I am looking at how survival differed between 3 groups of animals while taking into account that length, weight, and age may have played a role. When I look at these factors individually, none had a significant effect on survival...but, when I include interaction terms (i.e. length*weight, group*weight, etc.) the variables are significant. What does this mean? Am I wrong to put in my results that x,y, and z do not affect my results?

• Why do you mean by “pull the variables out individually”? Run an ANOVA with an interaction but no main effects? Run separate ANOVA with each factor in turn? – Gala Jun 15 '13 at 23:04
• Example: I ran a full model. A was sig, AD was sig, AE was sig, but B was not and C was not nor were any of the interaction terms that included B and C. Do I rerun the ANOVA with A, D, and E individually to see which variable actually influenced my results? – Michele Jun 15 '13 at 23:07
• You could conceivably leave B and C out but that should only affect things in specific instances. What kinds of sums of squares are are you using for your ANOVA? Do you know if there is any correlation among your predictors? What, specifically do you really want to find out. (It's better if you edit your question, not just add a comment.) – John Jun 15 '13 at 23:26
• I wish had a better grasp on stats, because I am still so confused. In essence, I am looking at how survival differed between 3 groups of animals while taking into account that length, weight, and age may have played a role. When I look at these factors individually, none had a significant effect on survival...but, when I include interaction terms (i.e. lengthweight, groupweight, etc.) the variables are significant. What does this mean? Am I wrong to put in my results that x,y, and z do not affect my results? – Michele Jun 16 '13 at 1:15