- Inclusion of negative control outcome/exposure; In similar studies, it has been suspected that there may be a risk of ascertainment bias, whereby parents of children who are delayed vaccinated, represent parents with a general lower health care seeking behavior, which could result in lower levels of AD diagnosis among their children. In order to accommodate this potential unmeasured confounder, the investigators will seek to identify a relevant negative control exposure or outcome, which will help indicate the strength of a potential interference from this unmeasured confounder.