A skill is a developed proficiency or mastery, an ability that has been improved by practice or training or both. The skill required to perform the task of moving from a starting place to a destination is such a composite skill. Prerequisite skills are skills that are not unique to the task but that must be developed before the task can be attempted. For example, a pedestrian must be able to walk or at least have some means of locomotion before attempting the mobility task. Necessary skills are those skills without which the task cannot be performed.
Obstacle avoidance is one example, and the actions it implicates are observable.
The use of landmarks to maintain orientation is another example, but the cognitive functions implicated by this skill are not directly observable and must be inferred from their behavioral consequences. 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.