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).
References
Lewis, James (2001) "
Evaluation
of Procedures for Adjusting Problm-Discovery Rates Estimated from
Small Samples" in
The International Journal of Human-Computer
Interaction 13(4) p. 445-479
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.