This calculator produces an estimate of the number of users needed to discover the specified percent of total problems. It uses the GoodTuring and Normalization procedure.
Sample Size from an estimate of Problem Occurrence (p)
If the probability of detecting a UI problem is known in advance, use this portion of the calculator to estimate the total number of users needed to uncover on average the specified percentage of problems (e.g. 90%). The calculator is based on the binomial probability formula.
Estimate Problem Occurrence (p) then Sample
Size
This portion of the calculator first builds
an estimate of the probability of detecting a UI problem (from sample
data). It then produces an estimate of the number of users needed
to discover the specified percent of total problems. It uses the GoodTuring
and Normalization procedure as outlined by Lewis (2001)
and further discussed in (Turner, Lewis & Nielsen
2006).



Lewis, James (2001) "Evaluation of Procedures for Adjusting ProblmDiscovery Rates Estimated from Small Samples" in The International Journal of HumanComputer Interaction 13(4) p. 445479
Turner, C. W., Lewis, J. R., and Nielsen, J. (2006). Determining usability test sample size. In W. Karwowski (Ed.), International Encyclopedia of Ergonomics and Human Factors (pp. 30843088). Boca Raton, FL: CRC Press.