1. Fit a regression line relating serum creatinine to time for each person in each of the high-NAPAP group, the low-NAPAP group, and the control group. How do you interpret the slope and intercept for each person? 

2. Use regression analysis or ANOVA methods to compare the slopes of the three groups. 

3. Answer the question in Problem 12.49 for the intercepts in the three groups. 

4. What are your overall conclusions regarding the comparison of serum creatinine among the three groups? (Note: One issue in the preceding analyses is that we have considered all subjects as yielding identical information regardless of the number and timing of visits available for analysis. A more precise approach would be to use methods of longitudinal data analysis to weight subjects according to the number and spacing of their visits. We discuss this in Chapter 13.) 

One issue in Problem 12.47 is that only the first and last visits are used to assess change in serum creatinine over time.