The last metric (word type) is slightly more conservative than<cite> Brent's (1999a)</cite> in that the word type must have been actually spoken in the same utterance (not the same block of 500 utterances) in which it was detected to count as a match. This lessens the possibility that a mismatch that happens to be segmentally identical to an actual word (but whose semantic context may not be conducive to learning its correct meaning) is counted as a match. However, this situation is presumably rather rare.