1. A data processing system, comprising computing hardware operable to perform operations comprising: processing input data instances through an ordered cascade of successive forward classification stages each configured to determine a respective likelihood score for each of the input data instances based on one or more attributes of the input data instance and to classify each of the input data instances by one of rejecting the input data instance, unconditionally accepting the input data instance, and conditionally accepting the input data instance based on the respective likelihood score, wherein in the processing the computing hardware causes the cascade to generate for each of the input data instances rejected by at least one of the forward classification stages a respective classification record indicative of classification into a rejected class, generate for each of the input data instances unconditionally accepted by all of the forward classification stages a respective classification record indicative of classification into an accepted class, and conditionally accept each of the input data instances that is conditionally accepted by at least one of the forward classification stages and is unconditionally accepted by all other ones of the forward classification stages; and generating for each of the input data instances conditionally accepted by the cascade a respective classification record indicative of classification into one of the rejected class and the accepted class based on the respective likelihood score determined by each of the forward classification stages for the input data instance.