16. The method of claim 15 , wherein the classifying comprises at each of the forward classification stages classifying each of the input data instances based on a comparison of the respective likelihood score with the respective set of first and second thresholds associated with the forward classification stage.
15. The method of claim 14 , wherein at each of the forward classification stages an input data instance is classified based on a respective set of first and second thresholds associated with the forward classification stage.