Entity_id is the primary key for the 'entities' table and one quarter of the primary key of the 'entity_values' table along with with measures.measure_id, Dateentity_values.date_start and Dateentity_values.date_end.
This key is used to link values in the 'entity_values' table to the particular physician, practice, nursing home, hospital or geographical location that is being measured.
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