Problem: Designing the user experience is a growing trend in product design; however this trend has not greatly benefited people with impairments and disabilities. There are no practical tools to broadly assist with this issue. There is a need for standardized measures to quantify impairment, a model to predict how designs may perform and a need for data regarding how people with impairments interact with consumer technology. Purpose: To conduct a usability analysis with an industry partner on their powered wheelchair controller using participants with varying impairments. The industry partner was seeking better insight into the benefits of formal user testing.
Method: Forty consenting adults were given a score representing their level of impairment using six measures from the International Classification of Functioning (ICF). These measures were identified by the researcher to affect interaction with a device. Performance was measured by time taken to complete tasks, errors made, reported task difficulty and reported controller usability. Results: Performance was reduced in participants with a higher ICF score and age. An ICF score less than or equal to 2 was 117 times more likely to not complete the tasks, greater than or equal to 3 was not able to complete the experiment. Age >50 years took an average 79 seconds longer than <35 years to complete a task and reported greater difficulty, more errors and a lower usability for the controller.
Implications: Low to moderate levels of impairment has a significantly negative effect on the usability of common devices. Difficulties were mostly cognitive with participants unable to create an accurate mental model of the system. Participants with lower performance tended to be overly optimistic about their abilities. Mistakes were the greatest source of error followed by lapses and almost no reported or observed slip errors. Original Contribution: The ICF has never been used as a metric for usability testing. This study successfully applied the ICF alongside other measures to prove its validity. Based on the results and current literature the Task Process Model was created to provide a simple and practical way to describe the interaction of people completing a task of basic to moderate complexity.