1. A computer-readable storage medium including instructions readable by a computer which, when implemented comprise: receiving a speech-based signal; generating a decoder result comprising at least one sequence of linguistic units based on applying a context free grammar to the speech-based signal; decomposing the decoder result into a decomposed decoder result comprising at least one sequence of subword elements; accessing a background model having probabilities of subword elements in one or more natural language; generating competitive sequences using the decomposed decoder result and the background model; and calculating a confidence measure using the competitive sequences and the decomposed decoding result by calculating a model based posterior probability score using the decomposed decoding result and the competitive sequences.