Hi and thanks for your help! Currently using SAS_UE (SAS Studio)   1) When computing hazard ratios with PROC PHREG using the HAZARDRATIO statement, both the Point Estimates and 95% Wald Confidence Limits seem to vary depending on which values are used as the 'Reference' values for the involved variables.   2) However, the HR's no longer vary with Reference changes if the interaction variable is removed from the model; when it is removed, the HR's are the same regardless of the reference values used. Not sure why that would be the case if I'm only calculating the HR at one value of Margins   Briefly:   Margins 0 = No surgery 1 = Negative margins 2 = Positive margins   Surgery 0 = No surgery  1 = Type1 surgery 2 = Type2 surgery   PROC PHREG data=work.import simple ; CLASS Grade Size Margins (Ref="No surgery")    /* Why does changing the reference value change the hazard ratios */ Surgery (Ref="None")          /* Why does changing the reference value change the hazard ratios */ / param=Ref; MODEL Time*Censor(1)= Grade Size Margins Surgery Surgery*Margins / selection=backward;   HAZARDRATIO Surgery / at (Margins="Positive") diff=all;   So ultimately, if I want to compare Type1 and Type2 surgeries when Margins="Positive" (and also compare when Margins=Negative): A) Do I even need the interaction variable? Why not just leave them separate and do HAZARDRATIO at Margins="Positive" B) Should I set the Reference values to Surgery="Type2" and Margins="Positive" because those values are evaluated in the HAZARDRATIO statement? or should I use reference values that are won't be assessed in the HAZARDRATIO statement (like Surgery="No surgery", and Margins="Negative" or "No surgery").   Tabulated the HR's, ran +/– interaction term and changing the reference values for Surgery and Margins Each HR here is from Results for [Surgery Type1 vs Type2 At Margins=Positive]  Intreraction? Surgery Ref Margins Ref HR Llimit Ulimit No interaction Type1 Negative 0.324 0.253 0.414 Surgery*Margins Type1 Negative 0.325 0.243 0.433 No interaction None Negative 0.324 0.253 0.414 Surgery*Margins None Negative 0.288 0.173 0.478 No interaction Type2 Negative 0.324 0.253 0.414 Surgery*Margins Type2 Negative 0.288 0.173 0.478 No interaction Type1 Positive 0.324 0.253 0.414 Surgery*Margins Type1 Positive 0.325 0.243 0.433 No interaction None Positive 0.324 0.253 0.414 Surgery*Margins None Positive 0.288 0.173 0.478 No interaction Type2 Positive 0.324 0.253 0.414 Surgery*Margins Type2 Positive 0.288 0.173 0.478   Thank you!! -D   ... View more