Measurement
CPA
Cost per acquisition
The advertising spend divided by the number of conversions attributed to it.
CPA = ad spend / attributed conversions
What makes it wrong.
The mechanism, not the warning.
CPA falls when a cheaper conversion event is added to the account, which looks like improvement and is arithmetic. A CPA compared across a period where the conversion configuration changed is comparing two different questions.
In detail.
CPA answers what one result cost. It is the inverse of the conversion side of ROAS and inherits every one of its attribution problems, with one addition: what counts as a conversion is a setting, not a fact.
Changing which events are marked as conversions changes every historical CPA the platform reports, without changing anything that happened.