Variety of movement and context characteristics the learner experiences. Increase in practice variability associated with increase error during practice. When error is in initial stages increase variability leads to better learning of task. Interference from practicing in different contexts. Random practice has high contextual interference.
Studies show high contextual interference lead to greater learning.
Look at graphs p 228, 232-233. Research evidence supports that learning through variable practice of different coordination patterns is better than learning related skills with only different parameter variations. If this is varied he must start from scratch and make a new action plan each time. This develops the person's problem-solving ability.