A list with two objects (a) X, a dataframe with n rows representing clinical subjects and columns:
treatment, x1, x2, x3, x4 and x5 where treatment is binary indicating the two arms of the clinical trial
and x1, ..., x5 are covariates that were collected about each subject and (b) y, a length n vector storing
the continuous response values where, in this mock dataset, larger values indicate "better" outcomes for the
subjects.