Help with model Matrix GLM


New Member
Hey guys been browsing the forums for a while, and usually find what I need! This is a tad more specific though so Ive made an account!! I've been puzzled by the form of the model for a while.. The model states that the response depends upon y~smoke + drink +smoke:drink +age:smoke. The model doesn't include Si,1 or di,1, or their interactions with the other dummy variables, until the last term age:smoke, where Si,1 is included. How come Si,1 is included in this last term? Also, how come we need Si,1 at all? since there are only two categories to "Smoke", surely if Smoke = 1, then Non Smoke = 0??
Any Help is much appreciated!