In the original tree, long dialogues are not penalized if they have achieved a complex task (i.e.
a number of studies have been done on predicting specific errors in a dialogue, using a variety of automatic and handlabelled features, such as ASR confidence and semantic labels (Aberdeen et al., 2001;Hirschberg et al., 2000;Levow, 1998;Litman et al., 1999).