I have a question on the coding/use of categorical variables: In short, I have two control variables in an SPSS regression model. One is participant (15 levels, or participants) and one other categorical variable (10 levels). And there is just one predictor variable in the model. I've tried two different approaches to the control variables:
1) Coding all 25 as separate 0,1 variables. This allows us to see more nuanced predictions, which is preferred!
2) Coding the two variables as two dummies (0, 1, 2, 3 etc), and (0, 1, 3, 4, etc.).
My question: I assumed I would get nearly the same result given this is essentially controlling for the same thing. But I'm seeing different results for the IV: a weakly positive coefficient for approach 1, and a weakly negative coefficient for approach 2. So my question is why is there a different result/sign reversal of a pretty weak predictor variables? This, to me, is weird.
I would really appreciate any input you may have on this - and what approach is correct - or if there may be something else wrong. Thanks so much!