Attribution
Cost per acquisition
Also called CPA, cost per conversion.
Cost per acquisition is media cost divided by the number of credited conversions. It tells you what each customer or lead cost in ad spend.
How it is measured
Add the ad spend for the period, then divide by the conversions credited to that spend under your attribution rule. The result depends on the rule and the window, so state both.
Include the right costs. Media-only CPA excludes agency fees and creative. A fully loaded figure includes them. Choose and label.
Worked example
A language-tutor marketplace spends $6,400 on a month of search ads and is credited with 160 sign-ups that book a first lesson. The cost per acquisition is $40.
A different attribution rule credits 118 of those sign-ups, so CPA becomes $54. The marketplace stays with the first rule for reporting but checks any budget move against the stricter one.
How it differs
Cost per acquisition divides by conversions. Cost per click divides by clicks. A cheap click that never converts makes a high CPA, which is the number that pays the bills.
Common errors
Using a different conversion definition each month. Mixing media and loaded costs. Ignoring the window. Comparing CPA across products with different values. Looking at CPA without lifetime value.
In practice
Set a target CPA from your margin, not from last month. Report CPA by campaign, not only in total. Compare it against what a customer is worth over time.