Low language capacity in childhood: A systematic review of prevalence estimates.

All 19 studies were cross-sectional, so they give a snapshot of how many children met a language cutoff at one point in time. They cannot show how children's language changes, or how many move in and out of difficulty. The studies used 14 different sets of diagnostic criteria and 26 different assessment tools, so the estimates are not measuring the same thing and the range between them partly reflects that. Most studies did not report how they chose their sample size (only three did), 13 of the 19 had potential selection bias, and three recruited by convenience, all of which could push an estimate up or down. The review counted only peer-reviewed studies published in English. Prevalence figures published in other languages, and unpublished work such as theses and conference abstracts, were not included. The reviewers could not check whether a study's analysis plan was set in advance, because observational studies have no registration system.

The authors state that they have no conflicts of interest to declare. The supplied text contains no funding statement, so it is not possible to say who paid for the review.

Do not take 25% as the real proportion of children with developmental language disorder. The top of that range came from studies using a one-standard-deviation cutoff or parent and teacher report, and the studies that applied explicit diagnostic criteria reported 3.3% to 7.6%. Do not compare the figures for younger and older children either. Different studies looked at different ages with different tools, so the lower rates in older children do not mean difficulties resolve.