What is the minimum sample size required to conduct a survey using the System Usability Scale (SUS) and the Technology Acceptance Model (TAM)?
With SUS it very much depends on project and user-group size and on what outcome you’re looking for, areas to act on, or areas to investigate further.
SUS, for most commercial concerns, a participant size of 40-50 will provide actionable results with a good level of confidence. For most small workplace projects, a sample of 5 can provide results that are effective enough to indicate areas for investigation.
TAM is a very different measurement and can vary significantly on which variation of TAM you’re using. TAM, is usually good enough for small user groups, but because of it’s disconnect from external influences can provide false positive and negative results.
TAM can be effective between 5 and 10 participants, due it’s general targeting of ethnographically-similar, smaller, user groups.
TAM2, and the evolving TAM3, with their greater recognition of external influencing factors, provide stronger, more nuanced results and are transferable to larger projects.
In most studies I’ve done or have seen, PU (Perceived Usability) and PE (Perceived Ease of use) correlations garner higher degrees of confidence between 40 and 70 participants.
It’s important to note that neither SUS nor TAM are diagnostic in nature, they are perceptual tools only. To identify actual usability pain-points user research and task-based testing will be of far more effectiveness.