Growth marketing
Incrementality
The sales that only happened because you advertised. The rest would have come anyway.
Definition
Incrementality is the difference between what happened with a campaign and what would have happened without it. If 100 people buy after seeing an ad, but 80 of them would have bought anyway, the incremental effect is 20 purchases. Those 20 are what you paid for.
The platforms' own reports usually count all 100. That is why retargeting and search ads on your own brand look so good. They reach people who were already about to buy. If you measure ROAS without asking about incrementality, it is easy to move budget to the channels that are best at taking the credit.
The way to measure it is to compare with a group that saw nothing. Switch off a channel in a few regions for four weeks and compare with the rest of the country. This is called a geo test or holdout. It is clumsy, costs some sales and is the closest thing to an answer key you will get.