There is a lack of agreement on the features used to differentiate Childhood Apraxia of Speech (CAS) from Phonological Disorders (PD). One criterion which has gained consensus is lexical inconsistency of speech (ASHA, 2007); however, no accepted measure of this feature has been defined. Although lexical assessment provides information about consistency of an item across repeated trials, it may not capture the magnitude of inconsistency within an item. In contrast, segmental analysis provides more extensive information about consistency of phoneme usage across multiple contexts and word-positions. The current research compared segmental and lexical inconsistency metrics in preschool-aged children with PD, CAS, and typical development (TD) to determine how inconsistency varies with age in typical and disordered speakers, and whether CAS and PD were differentiated equally well by both assessment levels.

Whereas lexical and segmental analyses may be influenced by listener characteristics or speaker intelligibility, the acoustic signal is less vulnerable to these factors. In addition, the acoustic signal may reveal information which is not evident in the perceptual signal. A second focus of the current research was motivated by Blumstein et al.'s (1980) classic study on voice onset time (VOT) in adults with acquired apraxia of speech (AOS) which demonstrated a motor impairment underlying AOS. In the current study, VOT analyses were conducted to determine the relationship between age and group with the voicing distribution for bilabial and alveolar plosives.

Findings revealed that 3-year-olds evidenced significantly higher inconsistency than 5-year-olds; segmental inconsistency approached 0% in 5-year-olds with TD, whereas it persisted in children with PD and CAS suggesting that for child in this age-range, inconsistency is a feature of speech disorder rather than typical development (Holm et al., 2007). Likewise, whereas segmental and lexical inconsistency were moderately-highly correlated, even the most highly-related segmental and lexical measures agreed on only 76% of classifications (i.e., to CAS and PD). Finally, VOT analyses revealed that CAS utilized a distinct distribution pattern relative to PD and TD. Discussion frames the current findings within a profile of CAS and provides a validated list of criteria for the differential diagnosis of CAS and PD.