I have a multilevel model built coinsidering repeated measures on students. Students performance may vary depending on study hours and tutoring hours before each exam.
Should I center the predictors around their grand mean (sample mean)? Should/can I also center the variable representing the time of measurement?
Is it correct to say that if I center all predictors the intercept represents the performance score of a student who has mean values of predictor? Or does it represent the average performance score of students, when predictors have mean values?
Lastly, is it ok to center both in the case that predictors are continuous and discrete variables?