1. A method of filtering phonetic units to be used within a concatenative text-to-speech voice comprising the steps of: receiving at least one phonetic unit that has been automatically extracted from a speech corpus in order to construct a concatenative text-to-speech voice; calculating an abnormality index for said phonetic unit, wherein said abnormality index indicates a likelihood of said phonetic unit being misaligned; comparing said abnormality index to a normality threshold; if said abnormality index does not exceed said normality threshold, marking said phonetic unit as a verified phonetic unit; and, building said concatenative text-to-speech voice using said verified phonetic units.