I'm new to statistics and I would greatly appreciate any help on this. I have a response (heart beat, a numerical variable) and five other independent variables. Four are numerical (Age, Weight, Calcium, and Vitamin1) and the fifth variable is the drug (0 mg, 10 mg, 20 mg). I'm confused on how I should treat the drug. I entered it as (0, 10, 20) and then fitted the following full model:
$y=b_0 + b_1Age + b_2Weight + b_3Calcium + b_4Vit1 + b_5Drug + b_6(Age \cdot Drug)+b_7 (Weight \cdot Drug)+b_8(Calcium \cdot Drug) + b_9(Vit1\cdot Drug)$
and did stepwise regression afterward. My question is: do I have to code drug as
factor(Drug)? What is the difference between using Drug as-is (0, 10, 20) compared to coding it as 0, 1, 2. Do I have to create two additional dummy variables for drug? Do I need the interaction term?
It is the idea or the concept and I don't understand. I would greatly appreciate any help.