Denominator
The “out of what” in any rate or share. Choosing it is part of the definition of the metric, not a technical detail.
Formularate = numerator ÷ denominator — both part of the definition
Try it · 30 seconds · from Atterna’s practice
“Your conversion rate is 52%.”
On Meridian, 52% of registered buyers paid at least once in three weeks, and 4.2% of visits ended in an order. Which is the conversion rate?
Preparing the practice…
How we work it through
- 1
Ask “out of what?” for every percentage.
- 2
Keep the same denominator when comparing periods.
- 3
Show the two raw numbers next to the rate.
On the practice dataset
The query we run on Meridian, Atterna’s practice marketplace, and what it returns.
-- 3,652 paying of 7,000 registered buyers;
-- 8,164 orders from 195,013 visits
SELECT 3652.0 / 7000 AS per_registered,
8164.0 / 195013 AS per_visit;Result52.2% per registered buyer against 4.2% per visit
The common trap
Comparing two rates with different denominators, or changing the denominator between periods.
When an AI answers
“Checkout rate fell from 5.1% to 3.1%.”
- What it didn’t know
- It compared returning visitors with new ones — two different denominators.
- Ask back
- “Are both rates out of the same kind of visitor and the same period?”
Read next
Examples use Meridian, Atterna’s fictional practice marketplace.