My question concerns the constant represents in logistic regression analysis (which can be included/excluded in SPSS).

I am trying to understand the effect of binary variable (risk factor) DP on the dependent variable RP, a symptom.

If I were to assume that the incidence of RP in the population W/O risk factor DP is 50%, then I believe I could leave the constant out of the model.

However if I do not "know" the incidence of RP in the population W/O risk factor DP, then I should leave the constant in. The model will then then include a negative constant to correct for a baseline incidence of <50%, or it will include a positive constant to correct for a baseline incidence of >50%. For example if the baseline incidence is 25%, the constant becomes -1.1.

The constant corrects for the probability/incidence of the dependent variable w/o ANY of the dependent variables (in my case I just have one dependent variabl)

Is this reasoning correct?

Thanks in advance