1. A speech processing apparatus comprising: a data store operable to store a plurality of word models; a receiver operable to receive an utterance; and a matching unit operable to determine which of a plurality of word models stored in said data store most closely matches an utterance received by said receiver, said matching unit being arranged to output a value indicative of the goodness of each match between a most closely matching word model and an utterance received by said receiver; characterized by further comprising: a confidence model store operable to associate each word model stored in said data store with data indicative of the probability of a said value indicative of the goodness of match being output if said word model correctly or incorrectly matches an utterance; and a confidence score output unit operable to determine and output a confidence score indicative of a calculated posterior probability that a received utterance has been correctly matched to a word model given that the match resulted in the output of a particular value indicative of goodness of match, said output unit being arranged to calculate said confidence score for a match utilizing said value output by said matching unit and said data associated with the word model matched to utterances by said confidence model store.