The study used a sample of Illinois children (born between January 1990 and March 1996) who were through age 3 continuously enrolled in Medicaid, a public health insurance program for low-income families. The study used "paid claims" data and ICD-9-CM health codes to identify children with one or more of three chronic conditions: chronic physical illness, developmental delay/mental retardation (dd/mr), and behavior/mental health conditions (b/mh)