More about the Binomial Probability Formula and Its Small Sample Adjustment 167 working in different usability testing spaces. The formulas developed over the past 25 years provide a principled way to understand the relationship between those spaces, and a better way for practi- tioners to routinely estimate sample-size requirements for these types of tests. HOW COMMON ARE USABILITY PROBLEMS? Websites Appear to Have Fewer Usability Problems Than Business or Consumer Software From the files of Jeff Sauro Just how common are usability problems in websites and software? Surprisingly there is very little out there on the frequency of usability problems. Part of the reason is that most usability testing happens early in the development phase and is at best documented for an internal audience. Furthermore, once a website is launched or product released, what little usability testing is done is typically more on benchmarking than on finding and fixing problems. I recently reviewed usability publications and a collection of usability reports from various companies. I only included tests on completed applications and live websites, excluding those that were in the design phase and didn't have current users at the time of testing. My investigation turned up a wide range of products and web- sites from 24 usability tests. Examples included rental car websites, business applications (financial and HR), and consumer productivity software (calendars, spreadsheets, and word processors). I didn't include data from heuristic evaluations or cognitive walk-throughs because I wanted to focus just on problems that users actually experienced. After adjusting the values of p for the various studies, I had data from 11 usability studies of business appli-