User Interface Dependability through Goal-Error Prevention

User interfaces form a critical coupling between humans and computers. When the interface fails, the user fails, and the mission is lost. For example, in computer security applications, human-made configuration errors can expose entire systems to various forms of attack. To avoid interaction failures, a dependable user interface must facilitate the speedy and accurate completion of user tasks. Defects in the interface cause user errors (e.g., goal, plan, action and perception errors), which impinge on speed and accuracy goals, and can lead to mission failure. One source of user error is poor information representation in the interface. This can cause users to commit a specific class of errors - goal errors. A design principle (anchor-based subgoaling) for mitigating this cause was formulated. The principle was evaluated in the domain of setting Windows file permissions. The native Windows XP file permissions interface, which did not support anchor-based subgoaling, was compared to an alternative, called Salmon, which did. In an experiment with 24 users, Salmon achieved as much as a four-fold increase in accuracy for a representative task and a 94% reduction in the number of goal errors committed, compared to the XP interface.