I want to perform a Cox Regression Analysis for the dependent variable of overall survival in years.
I want to use a categorical explanatory variable that has 3 levels. Let's say for the sake of simplicity it's 3 types of mutually exclusive radiological findings like:
- Good CT-Scan
- Neutral CT-Scan
- Bad CT-Scan
If my understanding is correct then for a given predictor the hazard in one group would be expected to be a constant proportion of the hazard in another group. So let's just say we pick Level 1 = Good CT-Scan to be the constant and then I get Hazard Ratios for "Neutral" and "Bad CT-Scans" when I run the Cox Regression Analysis in R.
My question is then, does it matter which level we choose to be the constant? How would I choose the level which needs to be constant?
R does this automatically, but I wonder if it makes any difference to switch.
Any help is appreciated.