# Cohort Definitions

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A cohort is defined using one or more **Concept Sets**. For example, one Concept Set could contain clinical diagnostics codes while another could contain administered drugs.
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Cohorts are created using **Cohort Definitions** that define entry, inclusion and exit criteria for **Concept Sets:**

* **Cohort Entry Events** (mandatory): enter people into the cohort by defining medical event codes using **Add Initial events**.
* **Inclusion Criteria** (optional): include (or exclude) people from the cohort by defining medical event codes using **New inclusion criteria**. By default, no criteria are defined.
* **Cohort Exit** (optional): remove people from the cohort. By default, people will be part of the cohort until the time of death or the end of the follow-up period (end of continuous observation).

Different cohort definition rules are needed for different Atlas Domains. FinnGen uses the following Atlas Domains and tables for medical event codes:

| **Domain**  | **Cohort definition rule** | **Table**             |
| ----------- | -------------------------- | --------------------- |
| Condition   | Add Condition Occurrence   | CONDITION\_OCCURRENCE |
| Procedure   | Add Procedure Occurrence   | PROCEDURE\_OCCURRENCE |
| Drug        | Add Drug Exposure          | DRUG\_EXPOSURE        |
| Measurement | Add Measurement            | MEASUREMENT           |
| Observation | Add Observation            | OBSERVATION           |
| Condition   | Add Death                  | DEATH                 |

### Topics in the section

* [Using the Death register in Atlas](/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/cohort-definitions/using-the-death-register-in-atlas.md)
* [Filtering by clinical registries in Atlas](/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/cohort-definitions/filter-by-clinical-registries-in-atlas.md)
* [Filtering by demographic criteria in Atlas](/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/cohort-definitions/filtering-by-demographic-criteria-in-atlas.md)
* [Defining exit rules for a cohort in rules](/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/cohort-definitions/defining-exit-rules-for-a-cohort-in-atlas.md)
* [Selecting the correct bo in Atlas for events and medical codes](/working-in-the-sandbox/which-tools-are-available/atlas/detailed-guide/how-to-define-a-cohort-in-atlas/cohort-definitions/mapping-between-atlas-registries-and-medical-codes.md)

### Upcoming topics

#### Complex cohorts with nested criteria

* How to define cohort with disease onset in a time range (NO INSTRUCTIONS FOR THIS YET)
* How to define cohort using medication intervals (NO INSTRUCTIONS FOR THIS YET)
* How to define cohort with seasonal rules (NO INSTRUCTIONS FOR THIS YET)
* How to import a standard cohort from OHDSI phenotype library (**NO INSTRUCTIONS FOR THIS YET**)


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