Can you please help me with 2 questions (both to do with dummy variables) -
1) If I have 2 sets of nominal variables (eg ethnicity [white, Indian, African] and smoking status [current smoker, ex-smoker, non-smoker]) and I turn each set into 2 dummy variables (+1 baseline reference variable for each set) for entry into a multiple linear regression in SPSS. How does SPSS know which baseline reference variable to use for which set?
At least in logistic regression you have to option to label each variable as categorical (so there seems to be some way of grouping them) - this does not exist for linear regression.
For my example given above, does it mean that the multiple regression would produce 2 constants (one for each set of dummy variables)?
2) If I recode ethnicity (White, Indian, African) into 3 dummy variables, why can't these be treated as 3 separate binary predictor variables and entered into the model? Ie why can't I enter 'White / Non-White', 'Indian / Non-Indian' and 'African / Non-African' : and thus why can't I interpret the resultant coefficients as White compared to Non-white, Indian compared to Non-Indian and African compared to Non-African?