I have a 2 level linear mixed effects model with Time nested within University. Time is treated as continuous and I am modeling this within SPSS. The outcome variable is the number of undergraduate degrees awarded in a STEM discipline.
At level 1, I have a number of continuous covariates that change from year to year (e.g., full-time enrollment, percent of students that receive federal aid, etc.).
I would like to grand-mean center all my covariates (both Levels 1 and 2), but how is the mean calculated for Level 1 time varying covariates? Do I simply use the mean across all time points, ignoring the nesting?