In the audience page, you have the option to select the percentage of the audience that is active, or eligible to see the insert. The remaining percentage of the audience will be used as a control group.
The control group is the group of users who fit the audience criteria but will not see the insert even if they fit the insert's trigger. The actual size of the control group is determined by the percentage you define and the number of users in that percentage that fit an insert's trigger.
For example, consider that you want to show users a survey when they click the Purchase button in your app. You target all 100,000 app users, but configure 45% of that group to be in the control group. Of the 45,000 potential control group users, only 1,200 tap the Purchase button while your survey insert is active. Your actual control group size is 1,200.
The control group can help you assess the relative effectiveness of an insert because you can see the results from users who saw an insert, versus those that fit the audience and trigger but did not see the insert.
Insert provides analytics that compare users who saw the insert to users in the control group. You can compare insert goal completion results to determine an insert's effect on specific user behavior (i.e. Was a user that saw an insert more likely to click on a certain button in the app versus the control group?), and you can compare general engagement metrics, such as average session duration, to determine the overall effect of an insert in your app.
Once a user is selected to be in the control group or in the active group, he will stay in this group.
It is recommended to start with a relatively large control group and decrease its size once you observe positive results from an insert. It is not recommended to enlarge the control group after the insert has started running.