In a drug experiment to boost immune response, 30 patients were selected and divided into four groups.Group i received dose level i of the drug, i=0, 1, 2, 3.Dependent variable is the immune response in the patients.

1. Describe a multiple regression model explaining all the predictor variables used
2. Provide an interpretation for the beta coefficients, including the intercept
3. If we wanted the intercept to represent the mean immune response for dose level 3, would the model need to be changed? If so, how?