The word “measure” in the preceding paragraph does not mean, either in the case of the machine or of the man, the gross dimensions of length, breadth, and thickness; these are equally immaterial, in most cases, whether the subject of measurement be a man or a machine. One measures a machine to determine its _capacity_ for certain work, and is little concerned about its characteristics that have no bearing upon those qualities that fit it for those particular duties. So the measurements of a human being whose capacity for certain duties is to be determined must be of those qualities which enable him or her to perform according to a certain pre-determined requirement. These qualities, in man, woman, or child, can be measured; not with the precision with which an engineer measures the parts of a machine that must fit within a thousandth of an inch, but with sufficient accuracy to determine quickly, inexpensively, and simply whether a given individual has the capacity to learn and perform any given task or class of work. To explain how these tests can be made, how science can be and is being substituted for guesswork in the selection of human beings for jobs and of jobs for human beings, just as science has displaced guesswork in the selection of material commodities, is the purpose of this book. Let us first point out clearly the difference between science and guesswork. The vast majority of jobs are filled by guesswork. The farmer who hires a field hand, the housewife who employs a cook, the foreman who takes on a new “hand” in the factory, and even employers hiring persons for more responsible positions, all do it, to a greater degree than they imagine, by guesswork. They may make inquiries, more or less thorough depending upon the compensation and responsibility involved, of persons who are reputed to know by observation something of the candidate’s qualifications. Unless the individual under consideration be flagrantly and patently unfit the reports thus obtained are almost always favourable. In many cases no effort is made even to obtain such reports. Many persons who regard themselves as intelligent employ men and women for all sorts of delicate operations and confidential and responsible relations as a result of observation alone; yet observation alone will tell no more about a man or a woman than it will about an automobile—the shape and the colouring.