Question: In a study where four levels of a magnetic resonance imaging (MRI) agent are each given to 3 cancer patients (so there are 12 patients in all), the response is a measure of the degree of seizure activity (an unpleasant side effect). The F -test for lack of fit to the simple linear regression model with X D agent level has a p-value of 0.0082. The t-tools estimate that the effect of increasing the level of the MRI agent by 1 mg/cm2 is to increase the level of seizure activity by 2.6 units (95% confidence interval from 1.8 to 3.4 units).
(a) How should the latter inference be interpreted?
(b) How many degrees of freedom are there for
(i) the within-group variation?
(ii) the lack-of-fit variation?