I would like to run a multiple linear regression between my protein of interest (continuous) and a separate protein (continuous). I would like to adjust for a covariate/confounder which has three levels (k=3), which is physician's diagnosis (control, mild cognitive impairment, Alzheimer's disease). From what I understand, in regression analyses, categorical variables with more than two levels must be dummy-coded (k-1). In this case, since my categorical covariate has 3 levels, I would need two dummy variables. The omitted/reference group would perhaps be individuals with a diagnosis of "control". Therefore, I would just enter those two dummy variables into the regression to control for them. Is this a valid approach to adjust for this covariate/confounder?