Another consideration is that the requirement of having X many days of observation before the diagnosis forces us to drop persons with shorter observation period before their diagnosis. This can be a problem in US data, where follow-up is truncated when persons change jobs or health plans. Palmsten et al showed this clearly in the context of drug safety in pregnancy using Medicaid (the observed effect is possibly more extreme than in other data sources); see figure 4 in Harnessing the Medicaid Analytic eXtract (MAX) to Evaluate Medications in Pregnancy: Design Considerations