This calculator produces an estimate of the number of users needed to discover the specified percent of total problems. It uses the Good-Turing 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 Good-Turing
and Normalization procedure as outlined by Lewis (2001)
and further discussed in (Turner, Lewis & Nielsen
2006).
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. 3084-3088). Boca
Raton, FL: CRC Press.
