We have also the means. The psychologists and physiologists who have measured fatigue rely almost solely upon output as the unit of measurement. Decrease in output in a comparable unit of time, and all other working conditions remaining the same, is taken as indicative of being the result of fatigue. The observed man who is measured may add introspections, he may tell how he feels while working and at the close of work; but this testimony of his, while interesting and worthy to be recorded with the other data, cannot be submitted to the accurate measurement of the observer. In applying fatigue measurement to the industries in the same way that we measure activity and what it produces, we try to discover at the same time the condition of the worker by his own accounts as to how he feels. We have not only conditions under which scientific observations can be made and a method of making them, we have also devices for measuring both activity and output and relative rate of output. Fatigue, a Test of Efficient Activity. As for the relation between fatigue and activity, practically all of our knowledge of fatigue is derived from our knowledge of the activity that produces it. We measure the activity itself, and its product. We then measure the interval of time that elapses before the organism has gained enough activity to perform the same work in the same amount of time and with the same results. A study such as this cannot extend over a short space of time only. It must be carried on until any fatigue that is accumulated shows itself; but it is simply a question of extending the time over which the experiment stretches, and of varying the length of rest periods until the desired information is recorded in the data. As we come to compare various activities and their results, we find that the fatigue is a measurement of the efficiency of the activity. If two methods of doing the same piece of work take the same amount of time and produce the same amount of output, and if the interval needed to recover from the second is longer than that needed to recover from the first, then, other conditions being equal, the first method is the more efficient. A close study of the variables that affect the two methods will be necessary to show exactly why the first method is more efficient than the second, but the excess fatigue certainly shows that it is more efficient. Fatigue can, then, be looked at in two ways: 1. As a product of doing work. 2. As a test of efficiency in doing work. The amount of work done and the product are affected by various elements which affect the activity. The Activity.