“…Features are selected on grounds of their supposed predictive power towards problems during the interaction between the system and its user. The employed features range from primitive attributes representing entities such as confidence scores output by the automatic speech recognition (ASR) module of the system (Hirschberg, Litman, and Swerts, 1999;Litman, Walker, and Kearns, 1999), lexical output of the ASR module of a SDS (Hirschberg, Litman, and Swerts, 1999;Van den Bosch, Krahmer, and Swerts, 2001), experimental parameters and identification of the underlying ASR grammar (Hirschberg, Litman, and Swerts, 1999;Litman, Walker, and Kearns, 1999), aspects of dialogue efficiency and quality (Litman, Walker, and Kearns, 1999), presence or absence of default assumptions, the amount of slots filled (Krahmer et al, 1999) or the system adaptivity (Hirschberg, Litman, and Swerts, 1999), to highly complicated features, involving a variety of semanticsbased attributes of the user input (Hirschberg, Litman, and Swerts, 1999;Litman, Walker, and Kearns, 1999;Walker, Wright, and Langkilde, 2000), and aspects of syntax in the user answer (Krahmer et al, 1999). As opposed to the primitive at-tributes, the latter types of features cannot be straightforwardly extracted from a SDS, which forms an obstacle for automatic, on-line error detection and recovery.…”