Undersampling Will Change the Base Rates of Your Model’s Predictions

In classification problems, the proportion of cases in each class largely determines the base rate of the predictions produced by the model. Therefore if you use sampling techniques that change this proportion, there is a good chance you will want to rescale / calibrate your predictions before using them in the wild.

Check out the full article at KDNuggets.com website
Undersampling Will Change the Base Rates of Your Model’s Predictions