Robert A. Virzi (1992)
Human Factors, 34(4), 457-468.
DOI: https://doi.org/10.1177/001872089203400407
Abstract. The empirical justification for "five users is enough" in formative usability testing — Virzi's data suggests the first five participants uncover ~80% of usability problems, with diminishing returns after. Frequently misapplied (it's about formative, not summative work), but foundational for the discount-usability mindset.
Tags: usability-testing five-user-rule foundational