Use the linear mixed model (also calledgrowth curve model) for children'sbone length data. Briefly, bone length (outcome) was measured at ages 8, 8.5, 9, and 9.5. We fit a linear mixed model to predict bone length using a random intercept and slope (for age), and fixed age, i.e.,

1. Write out the Ximatrix for this model.
2. Are the random intercept and random age term appropriate for this model?
3. If we add sex to the model, the covariate does not significantly predict bone

growth. Should we leave sex in the model? Why or why not?