Within the sequence of objectives in each area, the objectives should be grouped in to meaningful subsequences or units. Such units can be designated as representing different levels in progress and can provide break point so that when a student finishes a unit in that area, he or she may either go on the next unit in that area or may switch to a unit in an other area. ( For example, on completing Level B addition, the pupil may either go on Level C addition or move on to Level B subtraction.) ( Lindvall and Bovil,1966,p.3 )