A consideration of the time element involved in memorizing has given use to two other methods, the so-called concentrated and distributive. Given a certain amount of time to spend on a certain subject, the learner may distribute it in almost an infinite number of ways, varying not only the length of the period of practice, but also the length of time elapsing between periods. The experimental work done in connection with these methods has not resulted in agreement. No doubt there is an optimum length of period for practice and an optimum interval, but too many factors enter in to make any one statement. "The experimental results justify in a rough way the avoidance of very long practice periods and of very short intervals. They seem to show, on the other hand, that much longer practice periods than are customary in the common schools are probably entirely allowable, and that much shorter intervals are allowable than those customary between the just learning and successive 'reviews' in schools."[4] This statement leaves the terms very long and very short to be defined, but at present the experimental results are too contradictory to permit of anything more specific. However, a few suggestions do grow from these results. The practice period should be short in proportion as these factors are present: first, young or immature minds; second, mechanical mental processes as opposed to thought material; third, a learner who "warms up" quickly; the presence of fatigue; a function near its limit. Thus the length of the optimum period must vary with the age of the learner, the subject matter, the stage of proficiency in the subject, and the particular learner. The same facts must be taken into consideration in deciding on the optimum interval. One fact seems pretty well established in connection with the interval, and that is that a comparatively short period of practice with a review after a night's rest counts more than a much longer period added to the time spent the evening before.