How to measure variance in a classification dataset?
I would start with logistic regression, and get a measure of importance for each of your 20 predictor variables. It's a little tough to understand what you mean by 'variance' in your two classes of good and bad. If you mean variance in your predictor variables that lead to your response, you can calculate the variance of a linear combination of predictor variables (in your logistic regression model) using an appropriate theorem from Wikipedia