I hear that SPSS doesn't directly support a function for marginal effects in logistic regression. So I tried to do this manually and the results do make sense but I wanted to make sure they are actually correct.
I basically substracted the probability predicted by the model for a given individual from the predicted probability if the x was 1 unit higher. As an example if I wanted to get the marginal effect for β1 I'd do this:
individual Marginal effect of β1 = 1/(1+e^-(α+(x1+1)*β1+x2*β2))) - 1/(1+e^-(α+x1*β1+x2*β2)))
And then take the mean of all individual marginal effects. Just wanted to make sure if this makes sense and I'm computing the right thing here.
Also, am I right when I say it doesn't make sense to compute this for explanatory Dummy-Variables and I shoud use the Odds-Ratio instead?
I'm sorry for my poor English and very grateful for any comment on this.