arm_ex_12.3.1.R

# (b)  Use the same model fit to generate simulations of CD4 percentages at
# each of the time periods for a new child who was 4 years old at baseline.

# For that we have to first create the dataset with the hypothetical child:

# I'll assume 7 hypothetical dates
hyp_child <- data.frame(newpid = 120,
                        VDATE = sample(cd4$VDATE[!is.na(cd4$VDATE)], 7),
                        baseage = 4,
                        treatmnt = 1)
# The exercise doesn't say anything about the treatment variable but I have
# to assign a value to it because it's present in the previous model. I'll assume
# this new child was in the treatment.

year_pred <- predict(mod2, newdata = hyp_child)

# That's it! Now we have the predicted CD4 percentage for different time points
# for a hypothetical child.