You can use percentage splits to segment contacts in programs. Rules are not required, however, percent values must equal 100%.
The basic idea of a control group is simple. You can purposely exclude a random sample of contacts from a program to ensure that they do not receive messages. Then after your program completes, you can compare the activity or level of engagement of the control group (0) against those contacts that participated in the program, to determine the effectiveness of your campaign.
Connect a percentage split to the Audience icon, set to a 90/10 split. Allow the 90% to continue through the program and 10% to be held back or exited. If using an event-triggered Autoresponder program, place the control group after the first step.
Example: A re-engagement program
This example describes how to set a control group for contacts that participate in the program, so you can measure the success of the program, or a specific path within a program.
Target contacts who did not open an email in the last 6 months. Of the contacts that qualify for the program based on that rule, when they enter, hold out 20% and allow 80% to move forward through the program. You can then compare the open rate of the 80% that proceed through the program to the ones in the control group that receive no messages.
For more information about using the percent split feature, go here.