Repeated Measures ANOVA with a Continuous Variable?


New Member

I'm trying to run a repeated measures ANOVA. There's a continuous variable that's affecting my data, that I need to control for. I tried putting the continuous variable as a covariate, but unfortunately there was a significant interaction between the covariate and my grouping variable. Since this violates an assumption, I cannot use this continuous variable as a covariate. However, I somehow still need to account for the effects of this variable. Thoughts?
Am I right to assume that your covariate is expected to be associated with your grouping variable i.e. age (years) would be with *** on attractiveness

You would expect males and females would decline in attractivness as they grow older from age 18 to 65.... but there may not be any difference in attractiveness at age 18 between men and women, men become significantly more unattractive my age 25, whilst women remain only slightly less attractive as they did at age 18! But... by the age of 65 both men and women are as unattractive as eachother again!
(the effect of age on attractiveness is the same in men and women- however there is an iterraction as the interraction graph lines will not show the same response to age)

I had always thought that this sometimes happened, and although you may get a significant interraction its sometimes good idea to plot your interraction graph to see where the interraction lies.

Rather than using a covariate Try using your covariate as an additional factor? or multivariate regression?

Hope this helps?
Hi Lola,

I don't know of an assumption that says you can't have a continuous covariate interact with a grouping variable in repeated measures.

Can you provide more details about the model? Since it's repeated measures, I assume something is measured over time. Is the covariate also measured at multiple time points or just one? By your grouping variable, do you mean time (or whatever is repeated) or do you have a between-groups variable as well?