The i2b2 star schema in its initial setup defined a one fact per concept per patient approach to a medicial data warehouse design. The central i2b2 table observation_facts is a enhanced EAV (entity-attribute-value) structure with additional entity identifiers (patient, visit, provider). The visit dimension defines a (financial) encounter. This allows for the enhancement of the one-to-one relation (concept to fact) to a one-to-many relation by defining N financial encounters per patient. But source data often does not contain financial encounters and/or a fact bundling of much finer entities is needed.

With the addition of modifiers in i2b2 version 1.6 (see: Data Model of Modifiers in i2b2) a more useful functionality for a undefined (meaning: not just financial encounter related) one-to-many relationship was introduced. Modifiers offer a way of bundling concepts in in the i2b2 hierarchy and give these concepts a unique (for that patient and encounter) subidentifier in the observation_fact to connect facts of one patient.