What is an appropriate sample size? Usability testing is not the exhaustive exercise that some people expect it to be. A relatively small number of individual test sessions is usually sufficient to generate accurate and useful findings. One widely recognized study suggests that just five test sessions will generate many of the findings that could result from a much larger study. That said, the FDA expects summative usability tests to include at least 15 participants per distinct user group. Other regulators that recognize IEC 62366 as a proper guide for usability testing do not specify a sample size, but there is an implied need to meet the prevailing standard of care.