Development of ambulation was studied among 1,744 children, ages 3 through 10 years, with moderate to profound mental retardation. The sample was limited to children with some degree of mobility at initial testing. Evaluations over 6 years indicated that improvements in basic skills, although not probable, do occur far past the normative age for development of speech and ambulation and across all levels of mental retardation. The probability of improvement declined with age. Degree of mental retardation and severity of motor impairment had the expected negative effects on initial skill level and probability of improvement. Limiting the sample to subjects who were mobile minimized the effect of mortality.
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