How much change is true change? The smallest detectable difference of the Preschool Imitation and Praxis Scale (PIPS) in preschoolers with intellectual disabilities of heterogeneous aetiology

The teaching of imitation skills is often the first step in interventions for young learners with intellectual disabilities. The main goal of this study was to determine the smallest detectable difference (SDD) at 95% confidence of the Preschool Imitation and Praxis Scale (PIPS) in preschoolers with intellectual disabilities. Two raters independently scored videotapes of the imitation performance of 44 preschoolers (27 with Down syndrome, 10 with Non-Specific Mental Retardation and 7 with Low-functioning Autism) between 13 and 58 months of age (mean age 39.6 months, SD 11.9 months). Results revealed that the PIPS demonstrated acceptable interrater reliability on item level (weighted kappa values ranged from 0.52 to 0.96) and scale level (ICC = 0.986; 95% CI: 0.975–0.993). The SDD of the PIPS was 7.2%, indicating that the change score rated by different raters for an individual child with an intellectual disability is valid and that the PIPS can be used by early interventionists and researchers as an outcome measure to determine children's maturation or improvement.