2.2
Product-Market Fit / The 40% Test
A survey rule of thumb: ask users how they would feel if they could no longer use the product, and treat 40% answering “very disappointed” as the threshold for fit.
Sean Ellis (2009), from growth practice rather than from research
What it does
Gives a pre-scale team one number to argue about and a reason not to spend on growth until it moves. As a stop sign it is genuinely useful: it converts “do people want this?” from an argument into a measurement a team can repeat monthly.
When it breaks
The 40% threshold has no published derivation — it is a remembered benchmark from a small set of startups, not an estimate with an interval around it. Worse, the survey only reaches people still using the product, so it measures the enthusiasm of survivors and is structurally silent on everyone who already left. A product with a small, devoted, unscalable audience passes it comfortably; that is the exact failure mode it is meant to catch.
Case
Kromatic documents the false-positive pattern: teams clearing 40% among a narrow retained cohort, concluding they had fit, scaling spend, and finding the score collapse as acquisition reached beyond the initial audience — because the test never sampled the people who churned.
Kromatic — Why the 40% test gives false positives ↗Diagram — not yet drawn
A funnel with the survey sample drawn only across the narrow retained end, and everyone who left falling out of the wide end unmeasured — the 40% bar computed from the narrow slice alone.
In the wild
Unvetted · not part of the tier assessment
What has been written about this tool in the last twelve months. Machine-retrieved and unchecked — everything above this line was checked.
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