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There is a strong business case for automotive interfaces to undergo usability testing throughout their product development lifecycle. System Usability Scale (SUS) is a simple and standard measure of usability. To meet the timing needs for product development, usability testing needs to be performed in a quick, cost effective manner. Hence the required sample size of participants for a usability study is one of the critical factors. To determine an acceptable sample size, a Monte Carlo simulation using SUS scores from eleven different in-vehicle automotive interface usability studies was used to create 500,000 subsamples of different sample sizes. The percentage of subsamples with mean scores within the confidence interval of the population mean was calculated. At a subsample size of thirty-five, 95% of the subsamples have a mean SUS score within the 95% confidence interval of the population mean.

Measurement of usability with the System Usability Scale (SUS) is successfully applied to products in many industries. The benefit of any measurement scale, however, is limited by the repeatability of the associated testing process. For SUS, these factors can include sample size, study protocol, previous experience, and pre study exposure to the system being tested. Differences in user exposure can influence the usability assessment of interfaces which could affect the validity of SUS scores.