What is a key measure of usability related to effectiveness?

Effectiveness in usability refers to how well users can achieve their intended goals when interacting with a system or product. The measure related to effectiveness determines the extent to which users can successfully complete their tasks and reach their objectives. Completeness of achieving set goals signifies the success rate of users in accomplishing specific tasks without error. This reflects not only whether users can perform tasks but also whether they can achieve them adequately and fully, which is essential for evaluating the overall usability of a design.

Other measures, like time taken to complete tasks, the number of tasks completed, and user satisfaction ratings, while important, are more indicative of efficiency and affective responses rather than pure effectiveness. They provide insights into how suitable or enjoyable a system is but do not directly measure whether users can achieve their goals successfully. Hence, the completeness of achieving set goals is the most direct and relevant measure of effectiveness in assessing usability.