Repeated measures ANCOVA; covariate main effect and post-hoc tests

Question

Let's say I run a repeated measures ANCOVA, with valence (negative and neutral) as repeated measure and age as a covariate. What I want to know is how age affects my data.

1. I get a significant interaction between the covariate age and valence. As post-hoc I would do a linear regression, where I regress age on the difference between negative and neutral (negative minus neutral), since Anovas work on the difference between levels of the dependent variables.

2. I get a significant covariate effect (between subject table) and as a post-hoc I am doing a linear regression, regressing age on the mean of valence ((negative+neutral)/2).

My questions,

• First of all, is the approach to perform a repeated measures ANCOVA appropriate, since I am interested in the effect of the covariate and do not want to partial it out?
• Further, are linear regressions, as described above appropriate as testing the type of the relationship between the covariate and the variables that were significant?
• And last, is the way I would create the variable to be regressed on (mean for BS and difference between levels of the independent variable) how it should be? If not, on what should I regress the mean?

Thank you!!!!

Answer 1

For sure is too late, but you should work with linear mixed models instead of ancova or Lm. Your proposals include obtaining a new variable in both cases to consider it as a DV. With linear mixed models you dont need to do this ( and you can also include trial by trial data)

As a general example:

ID Condition value Age 1 neutral 6 35 1 negative 3.5 35 2 neutral 7 21 2 negative 2.3 21 .. . ...

Here, "value" is the average value of whatever you are using in each condition. In short, if you have this data structure you can easily analise it this way:

lmer(value ~ condition + age + (1|ID))

With this model you can test all that you said!