Holdout group

A holdout is a randomly chosen slice of the eligible audience deliberately excluded from a campaign or programme, so its behaviour shows what would have happened anyway. Comparing treated contacts against held-out ones turns an open rate into a revenue claim you can defend in a board meeting. Keep the selection random rather than convenient, size it so a revenue difference is detectable, and run a long-lived programme-level holdout alongside campaign ones — the campaign view hides cannibalisation between messages.