21. The method of claim 20 , wherein a unit comprises a specified set of trials in stage 3 directed to a respective stimulus passage at the current level.
21. The method of claim 20 , wherein a unit comprises a specified set of trials in stage 3 directed to a respective stimulus passage at the current level. 22. The method of claim 21 , further comprising: if there are no further trials to be performed in stage 3 at the current level, evaluating the unit to determine if the student passed the unit.