Information on the health of sampled children was collected in 1991 from both mother’s reports and medical records. The information spanned the child’s lifetime.

Mothers were asked to check whether they had ever been told their child had each of a list of 14 chronic health conditions, including asthma.

Medical records were obtained for all of the child’s visits through 1991. Medical providers had the option of filling the information for each visit into a survey form sent by the National Center for Health Statistics, or attaching a copy of the medical records for the sampled child. A doctor’s diagnosis of asthma was based on mention of asthma in either a checklist of possible conditions, or a listing of asthma by name or ICD-9 code (493.0) as a diagnoses for any visit.