Just how much should language change in a year?

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Our standardized tests show us skills children should have at each age/grade, and allow us to look at how they are performing compared to peers. However, they don't often provide us with empirically-supported expectations for language growth each year (particularly when typically-developing kids and those with developmental language disorders are lumped together in the sample). 
This study, however, pooled data from over 20,000 typically-developing kids (no joke—they pulled data from already-published standardized tests) and 497 kids with language disorders (this data came from two large studies underway, both including groups of kids receiving “standard” language intervention in the schools), to measure just how much change can be expected each year for each group.
What results are tables of effect sizes, one of typically-developing children and one for those with language disorders, for kids ages 3–9 (only up to age 7 for the group with language disorders, though). Three categories are represented: grammar, vocabulary, and overall language. The effect size values represent how much change can be expected in a year. Bigger numbers mean “more change”. Given this broad interpretation, a couple important trends can be seen in the tables:

  • For typically-developing kids, vocabulary and overall language grow most in the younger years, and grammar grows most in the early elementary years.
  • Children with developmental language disorder “… had growth similar to that of preschool children with typically-developing language across all language domains.” However, this slowed substantially by age five. So, “…despite receiving language intervention, language growth (for children with language disorders) may slow down at an earlier age.”

Presently, most of us use clinical experience plus individual client profile to predict expected annual growth in our students' language skills. Though you cannot use these tables to directly predict how much change will occur in an isolated skill, you can at least now predict which ages tend to garner the most growth for vocabulary, grammar, and overall language in general, which may be helpful in adjusting expectations.

Schmitt, M.B., Logan, J.A.R., Tambyraja, S.R., Farquharson, K., Justice, L.M. (2017). Establishing Language Benchmarks for Children with Typically Developing Language and Children with Language Impairment. Journal of Speech, Language, and Hearing Research. Advance online publication. doi:10.1044/2016_JSLHR-L-15-0273