Table 3.

Treatment allocation based on a natural experiment

Treatment allocation	Treatment allocation	Outcome assessment
Patent protected (before 2009)	Majority of patients receiving phenytoin (2007–2008)	Seizure frequency (2009–2010)
Patent expired (after 2009)	Majority of patients receiving levetiracetam (2010–2011)	Seizure frequency (2012–2013)

Notes: Measurement of prescription patterns before and after patent serves as one such natural experiment, in which changes in the effective antiepileptic-drug choice set are used as an independent instrument of treatment allocation. The natural experiment allows for identification of two groups using levetiracetam’s expiration year (2009): 1) patent protected – most patients received older drugs (eg, phenytoin), and 2) patent expired – most patients received newer drugs (eg, levetiracetam). The outcomes can now be assessed over a 2-year panel: 2009–2010 (patent protected) and 2012–2013 (patent expired).