Investigating the usability of assistive user interfaces. Interacting with Computers

A prototype e-mail system was developed for cognitively disabled users, with four different interfaces (free format, idea prompt, form fill and menu driven). The interfaces differed in the level of support provided for the user and complexity of facilities for composing e-mail messages. Usability evaluation demonstrated that no one interface was superior because of individual differences in usability problems, although the majority of users preferred interfaces which did not restrict their freedom of expression (free format). In contrast to traditional evaluation studies, no common pattern of usability errors emerged, demonstrating the need for customisation of interfaces for individual cognitively disabled users. A framework for customising user interfaces to individual users is proposed, and usability principles derived from the study are expressed as claims following the task artefact cycle