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Quantitative Usability, Statistics & Six Sigma by Jeff Sauro
Sample Size Calculator for Discovering Problems in a User Interface
by Jeff Sauro | October 1, 2006 :: 19 Related Questions
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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.
Input
Discover of all Problems.
Problem Occurrence (values between 0 and 1)

Results


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).

Input
Discover of all Problems.
Total participants
Problems Discovered:

Results
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.

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Related Questions

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How do you determine the sample size for data for which the mean and standard deviation are not known?
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Dear Stats Person, Here is my question. If a person were to guess a number from 1-10, the chance of guessing a 4 would be 0.1. If 5 people were asked to guess a number from 1-10, the chance of guessing a 4 would be 0.5. The chance of 2 people guessing a 4 would be (0.1)(01.)=(0.01), correct? Shouldn't the total number of people also be accounted for? That is what is the chance of 2 people guessing a 4 out of 8 people? or 2 people guessing a 4 out of 20 people guessing? What about the chance of 3 people guessing 4 out of 50 people guessing? Is there a formula for this? Thank you. Diane
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