- Subgroup analysis, which only includes first born children; Parents of children with older siblings may be more familiar with AD, which may result in a less frequent health care seeking behavior. Parents of children with older siblings, who have had AD themselves, may further have prescription medicines in the home from treatment of prior cases of AD among the siblings. Hence, cases of AD categorized based on disease specific prescriptions may be misclassified if the parents do not get a new prescription for that child.