In this study, a new assistive technology (AT) outcome measurement scale R-MATE (Retrospective Measurement of Assistive Technology Outcome) was developed and applied to the use of a personal computer as a writing tool for people with disabilities. R-MATE asks the user to evaluate their own skills and abilities, e.g. their self-efficacy and competency, in a retrospective manner bearing in mind their personal environment, tasks undertaken as part of their life events and introduction to assistive technology. Self-efficacy for writing was measured using R-MATE with thirteen patients who had Duchenne Muscular Dystrophy (DMD). The result showed that AT introduction increased the user's self-efficacy although it varied among the individuals and their life phases.