Incrementality
Also called Conversion lift.
Whether the conversions credited to a channel would have happened anyway. Measured properly by withholding ads from a randomised group and comparing outcomes.
Why it matters
It is the strongest available answer and the most expensive, because it means deliberately not advertising to some people for a while.
What goes wrong
A control group of non-channel leads is the cheap approximation of this and is honest about being one. Where the budget and the volume exist, a real geo or audience holdout beats it, and the two are worth using together: the cheap one continuously, the expensive one occasionally.
Next to this
The rest of this step
Measure this on your own data
The diagnostic reads a CRM export in your browser and reports your volume, your match rate and the spread between your leads against the thresholds in this glossary. Nothing is uploaded and no account is needed.