A promotion's gross sell-out lift over the account's own baseline, minus the dip in the weeks after when stocked-up shoppers stay away. The volume the promotion actually added, per account.
Without the post-window, every promotion looks good. A large gross lift with an equal dip moved volume forward and added nothing.
Each account against its own baseline. One baseline for all accounts makes small stores look flat and large stores look lifted.
Units sold in the promotional weeks minus the account's own baseline, minus the shortfall against baseline in the weeks after.
A two-week promotion sold 9,600 units against a baseline of 6,000: gross lift 3,600. The next three weeks ran 1,800 below baseline. Net lift is 1,800 units.
Stopping at gross lift. Every promotion looks good before the dip is subtracted.