I am running a regression with 1 cont. ind. var and 1 cat. ind. var. There are 3 levels to the cat var so I have the lowest level set as the dummy var. It is very apparent that as the cont ind. var increases, so does the Y, and as the level increases in the cat. ind. var, Y increases. If I run each of the x's seperately, I get coefficients that make complete sense. If I run them together, my cat. coefficients are slightly negative which doesn't make sense at all because as the level of the cat var increases Y also does. I am using my lowest level of the cat var as the reference group. Any suggestions/troubleshooting I should check?