Incrementality measures whether a sale attributed to a publisher would have happened anyway without that publisher's involvement, as distinct from simply capturing a sale the customer already intended to make. It's a growing concern in affiliate marketing, particularly with cashback and coupon publishers, who can sometimes claim commission on purchases that were already certain to occur.
An advertiser tests incrementality by holding out a control group of customers who see no coupon publisher link. If conversion rates are similar between the test and control groups, the coupon publisher likely added little incremental revenue beyond what would have happened anyway.
Affiliate attribution typically only proves a click occurred before purchase, not that the click caused the purchase. Proper incrementality testing needs holdout groups or geo experiments, which most affiliate programs aren't set up to run, making true incrementality measurement rare outside larger, more sophisticated advertisers.
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