Cohort Definition

The COHORT_DEFINITION table contains records defining a Cohort derived from the data through the associated description and syntax and upon instantiation (execution of the algorithm) placed into the COHORT table. Cohorts are a set of subjects that satisfy a given combination of inclusion criteria for a duration of time. The COHORT_DEFINITION table provides a standardized structure for maintaining the rules governing the inclusion of a subject into a cohort, and can store operational programming code to instantiate the Cohort within the OMOP Common Data Model.

More details about cohorts in The Book of OHDSI: https://ohdsi.github.io/TheBookOfOhdsi/Cohorts.html#Cohorts

Field	Required	Type	FK Table	Description
cohort_definition_id	Yes	bigint		A unique identifier for each Cohort.
cohort_definition_name	Yes	varchar(255)		A short description of the Cohort.
cohort_definition_description	No	text		A complete description of the Cohort definition.
definition_type_concept_id	Yes	bigint	CONCEPT	Type defining what kind of Cohort Definition the record represents and how the syntax may be executed.
cohort_definition_syntax	No	text		Syntax or code to operationalize the Cohort definition.
subject_concept_id	Yes	bigint	CONCEPT	A foreign key to the Concept defining the domain of subjects that are members of the cohort (e.g., Person, Provider, Visit).
cohort_initiation_date	No	Date		A date to indicate when the Cohort was initiated in the Cohort table.

Conventions

• The cohort_definition_syntax does not prescribe any specific syntax or programming language. Typically, it would be any flavor SQL, a cohort definition language, or a free-text description of the algorithm.
• The subject_concept_id determines what the individual subjects or entities of the Cohort consists of. In most cases, that would be a Person (patient). However, cohorts could also be constructed for Providers, Visits or any other Domain. Note that the Domain is not codified using the alphanumerical domain_id like in the CONCEPT table. Instead, the corresponding Concept is used. The Concepts for each Domain can be obtained from the DOMAIN table in the domain_concept_id .
• Sites submit the contents of their COHORT_DEFINITION table for safety checks only (to ensure that the correct definitions are held by each site). The list of all approved Cohort Definitions is maintained centrally. When a site requires a new Cohort Definition to be created, this must be communicated by other channels (e.g. e-mail). The technical team will create the record centrally and then share it to all participating sites.

Usage in HIC-CC

HIC-CC uses centrally-defined Cohort Definitions in the following situations:

• Opt-out: there will be one Cohort Definition for patients that have declared their intention to opt-out from all studies.
• Specific studies: in general, any study in HIC-CC can access all patients from the pseudonymised merged schema. If required, cohorts describing the population for specific research studies can eventually be created.