11 June 2026

Week zero is not a cohort

Product teams often label a column “W0” because the analytics tool did. That column is usually a calendar bucket: people who did something between Monday and Sunday. A cohort is stricter. It is a set of people who share a birthday you can defend in a room.

If the birthday is “first session,” a person who opened the app on a Tuesday and created an account on a Friday may sit in two different stories depending on who built the chart. If the birthday is “first paid event,” a free user never enters the table. Both choices are legal. Neither is implied by the word week.

The damage appears when someone compares this Monday’s W0 rate to last Monday’s and calls the difference retention. What moved may be the mix of birthdays, a deploy that created sessions without accounts, or a paid campaign that filled the bucket with window shoppers. The table did not lie. The caption did.

Write the birthday in a paragraph, freeze it for a quarter, and put the paragraph under the table. If you need to change it, start a new ledger rather than rewriting history. That habit is the first week of our Weekly Retention Ledger desk, because almost every pack we mark still skips it.

A calendar week remains useful for operations: staffing, incidents, spend. Keep it. Just stop asking it to be a population.