Furthermore, end-to-end systems like speech recognizers (Roark et al., 2004) and automatic translators (Och, 2003) use increasingly sophisticated discriminative models, which generalize well to new data that is drawn from the same distribution as the training data.
in many situations we may have a source domain with plentiful labeled training data, but we need to process material from a target domain with a different distribution from the source domain and no labeled data.