## Deriving a Problem Discovery Sample Size

###### By Jeff Sauro, PhD . August 3, 2008 . Tags: Usability Problem, Sample Size

Neilsen derives his "five users is enough" formula from a paper he and Tom Landauer published in 1993. Before Nielsen and Landauer James Lewis of IBM proposed a very similar problem detection formula in 1982 based on the binomial probability formula.[4] Lewis stated that: The binomial probability theorem can be used to determine the probability that a problem of probability p will occur r times