Linear Mixed-Model Analysis to Examine Longitudinal Trajectories in Vocabulary Depth and Breadth in Children Who Are Hard of Hearing

Abstract

Purpose Children who are hard of hearing (CHH) tend to have reduced vocabularies compared to children with normal hearing (CNH). Prior research on vocabulary skills in children with hearing loss has focused primarily on their breadth of knowledge (how many words are known). Depth of vocabulary knowledge (how well words are known) is not well documented for CHH. The current study used linear mixed models (LMMs) to investigate growth trajectories of vocabulary depth and breadth in CHH relative to age-matched CNH. Method Participants for this study included 155 children (93 CHH, 62 CNH) enrolled in a longitudinal study. Examiners administered a standardized measure of vocabulary knowledge at ages 7, 8, and 9 years. We constructed multiple LMMs with fixed effects for group and age. The models included various combinations of random intercepts for subject and item and random slope for age. Results For depth, CHH showed significant and stable deficits compared to CNH over time. For breadth, CNH showed greater vocabulary breadth, but the group differences diminished with age. For CHH, higher aided audibility, age, and maternal educational level were associated with greater vocabulary breadth and depth. Age at hearing aid fitting was not. Conclusions A major advantage of using LMM is that it allowed us to cope with missing data points while still accounting for variability within and across participants. Assessment of both vocabulary breadth and depth may be useful in identifying school-age CHH who are at risk of delays in language outcomes.