. Author manuscript; available in PMC: 2014 Dec 1.

Published in final edited form as: J Speech Lang Hear Res. 2014 Jun 1;57(3):850–864. doi: 10.1044/2013_JSLHR-L-12-0240

Table 1. Comparison of classifications of language impairment (LI) at 4 years of age.

Classification	Twins at 4 years		% males in LI group	Verbal score at 4 years		Maternal education		% with family history (1st degree) of language or reading difficulties
Classification	N	%	% males in LI group	M	SD	M	SD
Full sample	2,923	100.0	44.2	0.05	0.95	4.17	2.02	9.3
Psychometric LI	237	8.1	46.4	−1.79	0.41	3.57	1.92	11.0
Parent referral	171	5.9	54.4	0.13	0.71	4.27	2.12	14.0
Both psychometric LI and parent referral	110	3.8	70.0	−2.08	0.50	3.87	1.93	12.7
Neither	2,405	82.3	42.0	0.33	0.65	4.17	2.02	8.7

Note. Verbal score at 4 = age-regressed parent-reported expressive vocabulary z score; maternal education = level of maternal qualification, where 0 = none, 1 = General Certificate of Secondary Education (D, E, F, G grade) or equivalent school-leaving qualification, 2 = General Certificate of Secondary Education (A, B, C grade) or equivalent school-leaving qualification, 3 = A-level or equivalent university-entrance qualification, 4 = Higher National Certificate (tertiary vocational qualification), 5 = Higher National Diploma (tertiary vocational qualification), 6 = undergraduate degree, and 7 = postgraduate degree.