Recruiting participants
A key component of testing, sometimes overlooked, is the testers--the actual people who agree to participate in the testing effort. In the end, experience testing is all about real people--this is the theory, at least.
In practice, testing with real people is expensive and time consuming. Recruiting can be challenging when design schedules are tight, when relevant audiences are overseas or in remote places, and when the design team is small and limited in budget and time. As a result, many companies end up testing with a smaller set of actual people than initially envisioned.
The screener
Many products, such as smartphones, e-commerce websites, and cars are meant for a wide and general audience. Potentially, almost any person is thus a candidate to be recruited to participate in usability testing. However, a random call for volunteers might yield an enormous list of interested candidates, but no effective means to ascertain if they really qualify.
Some degree of qualification...
