In one embodiment, a method is provided. The method comprises receiving a query comprising a time period associated with events data stored in a longitudinal database. Using a processing system, a variable data structure is identified in view of the query. The variable data structure is used to identify attributes associated with the events data from the longitudinal database. A tree of operators is generated to evaluate the events data in view of the variable data structure and the time period. Thereupon, results corresponding to the query are provided by applying at least one operator of the tree of operators to the variable data structure. The results include a time series of outcomes related to the events data.