# Cohort Combine
*Please note: in order to use Cohort Browser on the Platform, an Apollo License is needed.*
## Cohort Combine Logic
Cohort combine logic allows you to combine existing cohorts with Boolean Logic operations
### **Overview**
Here is a summary of the functions for Cohort Combine
## **Rules and Boundaries**
* All cohorts must be from the same dataset.
* All cohorts must be saved before being combined.
* A cohort that is a result of combine cannot be combined a second time.
* Cohorts from different projects can be combined if they use the same underlying database.
* Cohort combine are very complex queries
* Beware of performance delays and timeouts as query gets more complex.
* Use extra caution when:
* Combining cohorts with genomic filters
* Combining cohorts with complicated filters
* Combining cohorts based on very large datasets
## **How To:**
1. Add your cohorts into the cohort browser by selecting “Load Saved Cohort” if the cohort has already been created and saved into the project, or “New Cohort” if a new cohort needs to be created. You can select up to 10 cohorts to load into the side menu.
2. Pick your cohort and add it to the browser. It will look like this.
3. At the bottom of the cohort tab, select "Combine Cohorts"
4. You will then have the following screen to combine. Pick your cohort combine logic, then select combine
## Examples of Cohort Combine Functions
### Intersection
#### **Overview:**
#### **Example:**
### Union
#### **Overview:**
#### **Example:**
### Subtraction
Important note: the order of the cohorts matters in this.
#### **Overview:**
#### **Example 1:**
#### **Example 2:**
### Unique
#### **Overview:**
#### **Example:**
### Complement
Important notes:
* Cohort must be saved before creating its complement (same rule as previous)
* A combined cohort (Intersection, Union, Subtraction, Unique) can be used to create a complement.
* A cohort created as a complement cannot be further used for combine / complement.
#### **Overview:**
#### **Example:**
