SYS/DEF
Conversion
The event you decided to count as a success. Until it is in the tracking plan, it is not a metric, it is a wish.
Definition
A conversion is a binary event: it happened or it did not, for a unit of analysis (session, user, account). Macro: purchase, demo request, signature. Micro: CTA click, scroll, account created. Both exist; only one should be the primary metric of a test, and it must be defined in the tracking plan with its id, deduplication, and window.
Without a stable unit, “+12% conversions” can mean more clicks, more sessions, or more users. Those are not interchangeable.
When it matters
Before you write the hypothesis, before you size n, before you wire the A/B tool. If two teams do not share the definition, the test is not reproducible.
It also matters for the expected value of a test: lift × conversions × value. A micro-conversion worth €0 directly does not fund an implementation sprint unless you explicitly tie it to a macro.
Pitfalls
Counting /thanks pageviews as purchases while a refresh replays the pixel. Mixing logged-in users and cookies. Promoting a micro-conversion to primary because it “moves faster”. Forgetting that the SEA conversion (click through to thank-you) is not the analytics conversion (purchase) — attribution drives through the gap.
Related terms
Related tool
Prioritise an experiment in money, from traffic, expected lift and the value of a conversion: Open expected value of a test.