Speech Development Between 30 and 119 Months in Typical Children I: Intelligibility Growth Curves for Single-Word and Multiword Productions

PURPOSE: We extended our earlier study on normative growth curves for intelligibility development in typical children from 30 to 119 months of age. We also determined quantile-specific age of steepest growth and growth rates. A key goal was to establish age-specific benchmarks for single-word and multiword intelligibility. METHOD: This cross-sectional study involved collection of in-person speech samples from 538 typically developing children (282 girls and 256 boys) who passed speech, language, and hearing screening measures. One thousand seventy-six normal-hearing naïve adult listeners (280 men and 796 women) orthographically transcribed children's speech. Speech intelligibility was measured as the percentage of words transcribed correctly by naive adults, with single-word and multiword intelligibility outcomes modeled separately. RESULTS: The age range for 50% single-word intelligibility was 31–47 months (50th–5th percentiles), the age range for 75% single-word intelligibility was 49–87 months, and the age range for 90% intelligibility for single words was 83–120+ months. The same milestones were attained for multiword intelligibility at 34–46, 46–61, and 62–87 months, respectively. The age of steepest growth for the 50th percentile was 30–31 months for both single-word and multiword intelligibility and was later for children in lower percentiles. The maximum growth rate was 1.7 intelligibility percentage points per month for single words and 2.5 intelligibility percentage points per month for multiword intelligibility. CONCLUSIONS: There was considerable variability in intelligibility development among typical children. For children in median and lower percentiles, intelligibility growth continues through 9 years. Children should be at least 50% intelligible by 48 months. SUPPLEMENTAL MATERIAL: https://doi.org/10.23641/asha.16583426