A Common Data Model (CDM) is a way of organizing data into standardized structures and observational content to enable alignment of patient data across multiple organizations. Each i2b2 ontology leverages the i2b2 Common Data Model (CDM), which is based on a "star schema": instead of separate tables for diagnoses, medications, and other data types, all patient observations are stored in a single "fact" table, and the ontology describes the different codes that are placed in this fact table.