Cost Per Acquisition CPA (aka CPA, "cost per action", eCPA)
What you paid, on average, for each conversion the ad platform counted.
The fork, why two teams get different numbers
"Acquisition" is undefined by the word itself, and that is the whole
problem. In an ad platform (Google Ads, Meta), CPA means cost per
CONVERSION ACTION, and the advertiser chose what that action is: it might
be a purchase, but it might be an add-to-cart, a lead form, or an app
install. Finance hears "acquisition" and assumes a paying CUSTOMER. So CPA
and CAC get set equal when they are not: CPA counts every conversion event
(a user who converts twice counts twice), CAC counts new customers. CPA is
also platform-attributed, each platform claims conversions inside its own
window, including view-through and modeled conversions, so summing CPAs
across Meta + Google double-counts the same sale.
The trap
Platform CPA looks great precisely because the platform is grading its own homework.
- The full trap, worked on a real export
- Every formula variant, spelled out
- The reconciliation anchor, what to tie it to and when to refuse
Reference: IAB (conversion measurement · reference) · MRC (attribution & IVT guidance · reference)