Computer-based system access by persons with disabilities: Differences in the effects of interface design on novice and experienced performance

It is well accepted that with even very simple tasks, a user's performance with a manual control device improves substantially over some period of time before stabilizing. Past studies with computer input devices have shown that the overall period of time required to learn to physically interact with a system is, however, generally quite short. For users with mobility impairments, however, not only may the overall physical learning phase be significantly longer than for nondisabled users, but certain features of the interface design may require a longer learning period than others. Depending on how different “initial” performance is from “practiced” performance, systems intended for “walk up and use” applications or casual use may need to be designed differently to allow easy access for persons with mobility impairments. Twenty persons with impaired hand and arm function (as a result of spinal cord injury) performed a target acquisition task with five manual computer input devices. Task completion times and errors were recorded. The results indicate that some devices and screen design features required significantly longer periods of time than others for the participants to become proficient at using them. The results also suggest not only that some physical design attributes negatively effect performance, but that the magnitude of the effects differ for “initial” performance and “practiced” performance. In fact, in some cases attributes which had no effect once performance had asymptoted were shown to have a significant effect on novice performance. The implications for interface design are discussed.