I am doing a logistic regression . My outcome is a categorical (yes/ no) pain after surgery. The predictors i wish to model for includes the type of anaesthesia , among other predictors. The problem is the types i wish to include are general anaesthetic, plain spinal anaesthetic, spinal anaesthetic with morphine and finally spinal anaesthetic with diamorphine. Now, the 3 spinal anaesthetic categories are not mutually exclusive unlike general anaesthetic versus spinal anaesthetic alone. Is this appropriate for logistic regression? Or should i be using spare predictors ; i.e. GA vs Spinal and then spinal vs spinal morphine vs spinal diamorphine.. Any advice (in plain language please) would be gratefully received.
What do you mean by they are not mutually exclusive? Can a patient be assigned in more than one anaesthesia category at the same time? No. A patient is assigned to the general anaesthesia, plain spinal anaesthesia, spinal anaesthesia with morphine or spinal anaesthesia with diamorphine category. One who is assigned to the plain spinal anaesthesia category cannot at the same time be assigned in the spinal anaesthesia with morphine category. This means each category is distinct. And your intent is to determine if the outcome differs by type of anaesthesia. So all you need to do is to choose one of the anaesthesia categories as your reference category and to run the logistic regression. Then the result of the analysis for each of the remaining three categories would be interpreted relative to the reference category. (However, if your objective is to just compare the outcome between patients receiving general anaesthesia and those receiving any type of spinal anaesthesia, then you can merge the three spinal anaesthesia categories and you can compare 'general anaesthesia vs spinal anaesthesia'.)