Incrementality
Incrementality measures the additional conversions that happened because of a marketing effort, versus what would have occurred anyway without it. It is usually tested with controlled experiments, such as holdout groups or geo tests, that compare an exposed audience against a comparable unexposed one.
Why It Matters
Standard attribution often credits channels for conversions that would have happened regardless. Incrementality reveals the true added value of spend, protecting budget from going to campaigns that merely take credit rather than create demand.
Common Mistake
Confusing attributed conversions with incremental ones. A channel showing many attributed conversions, like branded search, may add little on top of what customers would have done anyway, overstating its real contribution.