As the optimal back-off strategy can vary depending on the context, the realization model implements parallel back-off strategies (see steps 2, 4, and 5 in Figure 4(b))-that is, multiple back-off paths are explored at run-time, and the probability of each back-off node is computed as the maximum probability of all outgoing paths.
a major drawback of the utterancelevel overgenerate and rank approach is its inherent computational cost.