Jeff Sauro • October 1, 2006
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).



The New Face of Usability Testing
7 Techniques for Prioritizing Customer Requirements
How to Compute a Confidence Interval in 5 Easy Steps
The Experiment Requires That You Continue: On The Ethical Treatment of Users
The Five Most Influential Papers in Usability
97 Things to Know about Usability
A Brief History of the Magic Number 5 in Usability Testing
How common are usability problems?
5 Examples of Quantifying Qualitative Data
10 Things to Know about Usability Problems
Confidence Interval Calculator for a Completion Rate
Should you use 5 or 7 point scales?
8 Ways to Show Design Changes Improved the User Experience
What five users can tell you that 5000 cannot
Nine misconceptions about statistics and usability
How to Conduct a Usability test on a Mobile Device
Quantifying the User Experience: Practical Statistics for User Research The most comprehensive statistical resource for UX Professionals Buy on Amazon  
Excel & R Companion to Quantifying the User Experience Detailed Steps to Solve over 100 Examples and Exercises in the Excel Calculator and R Buy on Amazon  Download  
A Practical Guide to the System Usability Scale Background, Benchmarks & Best Practices for the most popular usability questionnaire Buy on Amazon  Download  
A Practical Guide to Measuring Usability 72 Answers to the Most Common Questions about Quantifying the Usability of Websites and Software Buy on Amazon  Download 