Recall the linear mixed model (also called growth curve model) we fit in class forchildrens bone length data. Briefly, bone length (outcome) was measured at ages8, 8.5, 9, and 9.5. We fit a linear mixed model to predict bone length using arandom intercept and slope (for age), and fixed age, i.e.,

a. Write out the Xi matrix for this model.

b. Are the random intercept and random age term appropriate for this model?

c. Recall when we added sex to the model in class, the covariate did not significantly predict bone growth. Should we leave sex in the model? Why or why not?