The infant monitor of vocal production (IMP) normative study: important foundations

The purpose of this study was to establish a set of normative data (growth curve and centiles) for the Infant Monitor of vocal Production (IMP) using a representative population of infants with typically developing hearing. A linear mixed effect model and regression was used to derive ‘stage-for-age’ trajectory and growth centiles from the standard sequence of IMP assessment scores of 85 infants with normal hearing (age range 3–13 months). A significant linear relationship was demonstrated between IMP scores and infant age (p < .001). No significant relationship was found between IMP scores and gender, mono/bilingual language environment, singleton/sibling status, maternal education, or maternal work status. Inter-rater reliability and correlation for agreement was strong (0.94). These findings show that IMP assessment depicts the vocal development of infants with normal hearing as an hierarchical relationship between the complexity of infant vocal productions and infant age. Normative gains in vocal competency (−1SD/+1SD) approximated one IMP question per month of age from an infant’s baseline level of IMP achievement.