How many testers are needed to assure the usability of medical devices?
Borsci, Simone and Macredie, Robert D. and Martin, Jennifer L. and Young, Terry (2014) How many testers are needed to assure the usability of medical devices? Expert Review of Medical Devices, 11 (5). pp. 513-525. ISSN 1743-4440
Before releasing a product, manufacturers have to follow a regulatory framework and meet standards, producing reliable evidence that the device presents low levels of risk in use. There is, though, a gap between the needs of the manufacturers to conduct usability testing while managing their costs, and the requirements of authorities for representative evaluation data. A key issue here is the number of users that should complete this evaluation to provide confidence in a product's safety. This paper reviews the US FDA's indication that a sample composed of 15 participants per major group (or a minimum of 25 users) should be enough to identify 90-97% of the usability problems and argues that a more nuanced approach to determining sample size (which would also fit well with the FDA's own concerns) would be beneficial. The paper will show that there is no a priori cohort size that can guarantee a reliable assessment, a point stressed by the FDA in the appendices to its guidance, but that manufacturers can terminate the assessment when appropriate by using a specific approach - illustrated in this paper through a case study - called the 'Grounded Procedure'.
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