Which KPI is indicative of user satisfaction with a system's usability?

The System Usability Scale (SUS) is a widely recognized tool used to assess the usability of a system, providing a subjective measure of user satisfaction. This scale consists of ten questions that cover various aspects of usability, allowing users to evaluate their experience with the system. The results from the SUS help identify how satisfactorily a system meets user needs and expectations, making it a direct indicator of user satisfaction.

In contrast, conversion rates focus on the percentage of users completing desired actions, which is more related to business outcomes than user satisfaction with usability. User error rates measure how often users make mistakes while interacting with a system, which can highlight areas of usability issues but does not capture overall satisfaction. Time on task tracks how long it takes for users to complete specific activities, which, while relevant to efficiency, does not directly address user satisfaction levels. Thus, the System Usability Scale stands out as the best option for evaluating user satisfaction in the context of usability.