4. To evaluate the effect of a certain drug on the bone density of rats, 12 rats were selected and injected with the drug. The bone density (mg/cm2) of the rats over time was recorded. The results are given in Table 10.26.

Please answer the following questions:

(a) What is the main purpose of this design?

(b) What do normality and homogeneity of variance imply in a design with such repeated measures? How do you interpret the sphericity assumption in this case? Test these assumptions.

(c) Use appropriate methods to analyze the data and interpret the results.

(d) If necessary, use appropriate methods to conduct multiple comparisons.