Objective and subjective approaches to the force and displacement characteristics of input devices used by the disabled

This study was designed to determine how well therapists were able to estimate the mechanical characteristics of adapted switches. This was accomplished using two related experiments. In the first experiment, objective data characterizing the activation force-displacement trajectories for eight commonly used adapted switches were collected. In the second experiment, subjective data identifying the relative ranking of activation force as well as activation displacement by experienced and novice therapists were compiled. The major finding was that although therapists' subjective estimates of activation force and displacement were reasonably good there were specific areas of weakness that should be rectified with quantitative, objective data. In particular, subjective estimates of the mechanical properties appeared to be inadequate when used to evaluate switches separated by only small differences in force or displacement and for those whose properties were larger or smaller than their expected values. Possible difficulties in dissociating activation force and displacement were also noted. The technical and clinical implications of these results are discussed.