Question: In an experiment, rats were fed a stock ration for 100 days with various amounts of gossypol added. The following weight gains in grams were recorded.
![269_Gossypol.png](https://secure.tutorsglobe.com/CMSImages/269_Gossypol.png)
Suppose we assume the normal regression model for these data and treat gossypol as a categorical predictor taking five levels.
(a) Create a side-by-side boxplot graph for the data. Does this give you any reason to be concerned about the assumptions that underlie an analysis based on the normal regression model?
(b) Produce a plot of the standardized residuals against the factor gossypol (if you are using a computer for your calculations, also produce a normal probability plot of the standardized residuals). What are your conclusions?
(c) Carry out a one-way ANOVA to test for any differences among the mean responses for the different amounts of gossypol.
(d) Compute 0.95-confidence intervals for all the pairwise differences of means and summarize your conclusions.