Thank you for your quick reply. In the UG example 6.10, the following syntax is used: i s on X1 X2; This is described as regressing the growth factor on X1 and X2. How is this the same as regressing the intercept (mean) random effect on X1 and X2?
In the above question, I used the term intercept when I should have used the term mean since i and s are latent variables. Thank you.
"Growth factor" is the term we use for the random effects. The intercept random effect is i. So i is a latent variable. It has a mean and a variance and since it is a variable it can be part of a regression. Let me know if that is not what you are asking.