I believe to take a sample containing 80% observations, if you want to touch target variables classes equally then you must try what i mentioned above, else you can start with this in order Train:Validation:Test as 1: 40:30:30 (SAS default) 2: 45:25:30 3. 50:25:25 4. 50 :30:20 like this you can try and see what set gives you a better generalized model.
... View more