I have a dataframe ('math') like this (there are three different methods, although only one is shown) - dataframe
I am trying to create a multi-level growth model for MathScore, where VerbalScore is an independent, time invariant, random effect.
I believe the R code should be similar to this -
random <- plm(MathScore ~ VerbalScore + Method, data=math, index=c("id","Semester"),
model="random")
However, running this code results in the following error:
Error in plm.fit(object, data, model = "within", effect = effect) :
empty model
I believe it's an issue with the index, as the code will run if I use:
random <- plm(MathScore ~ VerbalScore + Method + Semester, data=math, index="id",
model="random")
I would be grateful for any advice on how to create a multi-level, random effect model as described.