What is a holdout group? How do I set one up?

A holdout group allows you to randomly withhold a custom percentage of your list or segment from mailing. When configured, you're able to download the data on both the holdout and mailed group, allowing you to conduct a clear analysis on the incremental lift from the mailing. To set up a holdout group, you’d simply enable the Holdout option during campaign creation and select the percentage you’d like to hold out from being mailed.