Plans I and II are obviously inferior to Plans III and IV; and Plan IV gives promise of being more effective than Plan III, since there seems danger that the pupil working by Plan III might in the ten weeks lose too much of what he had gained in the initial practice, and so again in the next ten weeks. It is not wise, however, to try now to make close decisions in the case of practice with division by a fraction; or to determine what the best distribution of practice is for that or any other ability to be improved. The facts of psychology are as yet not adequate for very close decisions, nor are the types of distribution of practice that are best adapted to different abilities even approximately worked out. [Illustration: FIG. 7.--Plan I. 200 practices distributed somewhat evenly over 3-1/2 years of 10 months. In Figs. 7, 8, 9, and 10, each tenth of an inch along the base line represents one month. Each hundredth of a square inch represents four practices, a little square 1/20 of an inch wide and 1/20 inch high representing one practice.] [Illustration: FIG. 8.--Plan II. 200 practices distributed haphazard over 3-1/2 years of 10 months.] [Illustration: FIG. 9.--Plan III. A learning period, three reviews, and incidental practice.] [Illustration: FIG. 10.--Plan IV. A learning period with reviews of decreasing length at increasing intervals.] SAMPLE DISTRIBUTIONS Let us rather examine some actual cases of distribution of practice found in school work and consider, not the attainment of the best possible distribution, but simply the avoidance of gross blunders and the attainment of reasonable, defensible procedures in this regard. Figures 11 to 18 show the distribution of examples in multiplication with multipliers of various sorts. _X_ stands for any digit except zero. _O_ stands for 0. _XXO_ thus means a multiplier like 350 or 270 or 160; _XOX_ means multipliers like 407, 905, or 206; _XX_ means multipliers like 25, 17, 38. Each of these diagrams covers approximately 3-1/2 years of school work, or from about the middle of grade 3 to the end of grade 6. They are made from counts of four textbooks (A, B, C, and D), the count being taken for each successive 8 pages.[10] Each tenth of an inch along the base line equals 8 pages of the text in question. Each .01 sq. in. equals one example. The books, it will be observed, differ in the amount of practice given, as well as in the way in which it is distributed. [10] At the end of a volume or part, the count may be from as few as 5 or as many as 12 pages.