These five figures are typical of nearly all {illust. caption = FIG. 5.} practice, or learning, curves. They depict the rate at which the beginner increases his efficiency. In every case we discover very great <p 232> fluctuations. On one day or at one moment there is a sudden phenomenal improvement. The next day or even the next moment the increase may be lost and a return made to a lower stage of efficiency. There are certain forms of skill which cannot be acquired rapidly in the beginning. In such instances a period of time is necessary in which to ``warm up'' or in which to acquire the knack of the operation or the necessary degree of familiarity and self-confidence before improvement becomes possible. This is true particularly in the ``breaking in'' of new operators on large machines like steam hammers, cranes, and the like, where the mass and power of the machine awes the new man, even though he has had experience with smaller units of some kind. It applies also to new inspectors of mechanical parts and completed products in factories--especially where the factor of judgment enters into the operation. Such instances are exceptions, however, and differ from those cited only in having a period of slow advance preliminary to the rapid progress. <p 233> Apparently, improvement should be continuous until the learner has entered into the class of experts or has reached his possible maximum. As a matter of fact the curve which expresses his advance towards efficiency never rises steadily from a low degree to a high one. Periods of improvement are universally followed by stages of stagnation or retrogression. These periods of little or no improvement following periods of rapid improvement are called ``plateaus'' and are found in the experience of all who are acquiring skill in any line. These plateaus are not all due to the same cause. They differ somewhat with individuals and even more with the nature of the task in which skill is being acquired. With all, however, the following four factors are the most important influence:-- 1. _The enthusiasm dependent upon novelty becomes exhausted_. 2. _All easy improvements have been made_. 3. _A period of ``incubation'' is needed in_ <p 234> _which the new habits under formation may have time to develop_. 4. _Voluntary attention cannot be sustained for a long period of time_. These four factors are not only the causes of the first plateau, but, as soon as any particular plateau is overcome and advance again begun, they are likely to arrest the advance and to cause another period of recession or of no advance. These four factors are therefore most significant to every man who is trying to increase his own efficiency or promote the progress of others. _When the interest in work is dependent on novelty, the plateau comes early in the development, and further progress is possible only by the injection of new motives to action_.