Desirable skills are skills that, although they enhance performance of the mobility task, are not necessary for its performance. The skill required to walk at a speed that does not cause interference with other pedestrians who are performing the same task, at the same time, and in the same setting is an example of a desirable skill. The elimination of interference is not a condition on which task performance depends. If the blind pedestrian performing the task walks at a much slower speed, he or she may interfer with other pedestrians, and the cost in time to the blind pedestrian may be considerable, but the task can still be performed. The task confronting blind pedestrians, as we have suggested, is the task of independently traveling from a starting place to a destination.
Successful mobility involves much more, however, than just the act of reaching a destination.
Measures are needed to determine the success with which the blind pedestrian has performed the mobility task. These measures involve the development of performance criteria. Performance Criteria A performance criterion is a standard against which a performance to be evaluated is compared. If the performance to be evaluated is equal to or greater than the standard, the criterion has been met. More commonly, a criterion denotes a level of performance that marks the boundary between adequate and inadequate performance.