Metrics
CPA (cost per acquisition)
CPA is the average media cost of one desired outcome - a purchase, lead or sign-up - calculated as spend divided by conversions. It is the workhorse efficiency metric of performance media, meaningful only when the conversion event and attribution rules behind it are clearly defined.
In practice
Two teams can report different CPAs from identical delivery because they count different events or attribution windows. Define the event, the window and the source of truth in the plan.
Watch marginal CPA as spend scales, not just the blended average - the average always lags the deterioration.