arm_ex_12.2_2.R

# (b) Extend the model in (a) to include child-level predictors
# (that is, group-level predictors) for treatment and age at baseline.
# Fit using lmer() and interpret the coefficients on time, treatment,
# and age at baseline.

mod2 <- lmer(CD4PCT ~ VDATE + treatmnt + baseage + (1 | newpid), data = cd4)
display(mod2)

#            coef.est coef.se
# (Intercept) 67.28     9.82  
# VDATE       -0.01     0.00  
# treatmnt     1.26     1.54  
# baseage     -1.00     0.34  
# 
# Error terms:
#   Groups   Name        Std.Dev.
# newpid   (Intercept) 11.45   
# Residual              7.32   
# ---
#   number of obs: 1072, groups: newpid, 250
# AIC = 7906.3, DIC = 7878.8
# deviance = 7886.5