Jakob Nielsen (1994)
International Journal of Human-Computer Studies, 41(3), 385-397.
DOI: https://doi.org/10.1006/ijhc.1994.1065
Abstract. Nielsen's foundational five-user-rule paper — empirical derivation of why 5 users find ~85% of usability problems in think-aloud testing. The single most-cited reference for discount usability and the canonical citation in any "how many users" debate.
Tags: design-heuristics usability-testing five-user-rule foundational