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Definition
Incrementality is the extra result caused by ads, compared with what would have happened without them. It is measured with a control, not read from a dashboard.
Example with made-up numbers. A test group of 50,000 people places 600 orders and a comparable holdout group of 50,000 places 500. The extra 100 orders are the lift attributed to the ads.
How to use it
Attribution tells you which ad was seen before an order. It does not tell you whether the order would have happened anyway. Incrementality asks that second question.
The usual method is a holdout test: show ads to one group, withhold them from a comparable group and compare outcomes. Results depend on the design, the size and the period, so state all three.
