index event; that is, the final data set contained 1 record per day per person, with the units of observation being person-days. Our decision to choose a 2-year maximum observation window was guided by the mean length of time individuals were observed in our data set, which was approximately 2 years. The selection of a standard, 2-year observation window also helps fulfill the case-control design’s requirement of a stable within-person probability of exposure over a defined period.21
