Concern About the Validity of These Norms as Representing a Population Sample

I am concerned that the milestone data in this paper1 are not based on a representative population sample. The data published for 75% of the population reaching the milestones listed in Tables 3 and 5 seem to reflect a data set based on the most delayed 20% to 25% of the population with Down syndrome. It seems that this clinic population is skewed to those with more medical issues, more severe developmental delays, and behavioral difficulties. I believe these milestones to be misleading, risk lowering practitioner expectations, and could cause harm to this population and their families.There are some very odd contradictions apparently not noticed (eg, “Use of 50 or more words, signs, or pictures was predicted to be achieved by 30% of children by age 4.2 years and 90% by 21.3 years. Two-word phrases were predicted to be used by 30% of children by 2.0 years and 90% by 11.4 years.”) Children do not use 2-word phrases until they say 50 to 100 words. Does this suggest the method of data analysis needs reviewing?Here are a few published examples from the very extensive literature on language and reading progress for children with Down syndrome to support my view. They are smaller but community-based samples.Galeote et al2 identify that 96.67% of children with Down syndrome are combining words at CA 39 to 71 months, mental age 26 to 29 months. Foster-Cohen et al3 report that at 42 to 66 months CA vocabulary sizes are 53 to 480 words. Other teams have similar data on spoken vocabulary and grammar development.On reading development, Baumer et al1 report, “The ability to read 10 or more words was predicted at 15% probability by age 7.7 years, 30% by 10.3 years, 45% by 11.8 years, 60% by 13.0 years, 75% by 14.2 years and 90% by 16.7 years.”Hulme et al4 report on the longitudinal reading progress of 49 children with Down syndrome mean age 124.71 (SD 36.7). At T1, the word reading raw score on a standardized reading test of increasing difficulty (so not their total sight word vocabulary, which would be much larger) was mean 49.4 words (SD 36.97) and at T3 63.68 (SD 33.92). Word reading standard scores ranged from 55 to 115 with 4 children word reading at their CA level.Burgoyne et al5 with a younger group (N = 57, age 5–11 years) at T1 report word reading also on a standardized test T1 5.86 (SD 10.41, range 0-46), T2 10.50 (SD 12.01, range 0-52), and T3 14.86 (SD 14.02, range 0-55). Importantly, 48 of 53 children were able to score on the reading test at T3 (compared with 32 at T1), and 10 children attained a standard score of 90 or above on the reading measure at this time.I am also concerned about the rationale for the article. We know a great deal about the expected impacts of Down syndrome on development—we do not need to wait or consider any “outcome norms.” We should provide interventions to support all aspects of the child’s development from the time of diagnosis.