Purpose: Three previous articles provided rationale, methods, and several forms of validity support for a diagnostic marker of childhood apraxia of speech (CAS), termed the pause marker (PM). Goals of the present article were to assess the validity and stability of the PM Index (PMI) to scale CAS severity. Method: PM scores and speech, prosody, and voice precision-stability data were obtained for participants with CAS in idiopathic, neurogenetic, and complex neurodevelopmental disorders; adult-onset apraxia of speech consequent to stroke and primary progressive apraxia; and idiopathic speech delay. Three studies were completed including criterion and concurrent validity studies of the PMI and a temporal stability study of the PMI using retrospective case studies. Results: PM scores were significantly correlated with other signs of CAS precision and stability. The best fit of the distribution of PM scores to index CAS severity was obtained by dividing scores into 4 ordinal severity classifications: mild, mild-moderate, moderate-severe, and severe. Severity findings for the 4 classifications and retrospective longitudinal findings from 8 participants with CAS supported the validity and stability of the PMI. Conclusion: Findings support research and clinical use of the PMI to scale the severity of CAS.