The i2b2 star schema in its initial setup defined a one fact per concept per patient approach to a medicial medical data warehouse design. The central i2b2 table observation_fact is an enhanced EAV (entity-attribute- valuevalue) design with additional entity identifiers (patient, visit, provider). The visit dimension defines a (financial) encounter. This allows for the extension 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 an 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, encounter and modifier path) subidentifier sub-identifier in the observation_fact to connect facts of one patient. 

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In this example the patient with the patient_num “1” has two specimen specimens with three attributes: the specimen ID, the amount left and the type of specimen as an SPREC code. Although a query with the i2b2 Webclient Web Client for serum specimen with an amount_left higher than 0.1 will return patient “1” with all specimens, the IDRT i2b2 Webclient Plugin Web Client plugin is able to show only secondary entities of patients which really match the query.

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