1. A method of assessing a key child's expressive language development, comprising: processing an audio recording taken in the key child's language environment to identify segments of the recording that correspond to the key child's vocalizations, wherein a computing device configured to perform the processing is used and the processing includes categorizing a plurality of segments of the audio recording into a plurality of categories, the plurality of categories including categories selected from the group consisting of vocalizations, cries, vegetative sounds, and fixed sounds, and determining which of the plurality of segments characterized as vocalizations are segments of the recording that correspond to the key child's vocalizations by comparing the plurality of segments characterized as vocalizations to a plurality of models; applying an adult automatic speech recognition phone decoder to segments of the key child's vocalizations to identify each occurrence of each of a plurality of phone categories, wherein each of the phone categories corresponds to a pre-defined speech sound; determining a distribution for the phone categories; and using the distribution in an age-based model to assess the key child's expressive language development.
5. The method of claim 1 , wherein there is a correlation between the frequency of each of the phone categories to chronological age.