Hello,
I was looking at the documentation of 3dLME, trying to understand if this is what I need for my own data:
[afni.nimh.nih.gov]
My data also has RT per condition per subject, similar to Example2. Originally, I was thinking of adding RTs as a predictor to the analysis, but then I reckoned it was a bad idea as it correlates with the Condition predictor. I see that in Example2 in the documentation, RT is added as a random effect. What is the meaning of RT being a random effect?
Perhaps I am confused from what I know about mixed models in regular behavioral experiments: usually the random effect is subjects (or any other random grouping criterion), because that's randomly sampled from the population. How is RT a random grouping variable?
In analogy, an R formula would look something like this:
lmer(BOLD ~ Cond*RT + age + (Cond|RT))
And this I don't understand conceptually. I suppose my concept is wrong. I would very much appreciate your help in understanding this.
Thanks,
Galit