The things which are not so evident at a glance are the degrees of relationship between these three types of measures. Is the relation of educational measurements to the teacher’s estimates greater than the relation of the measurements to the marks in scholarship given by the teacher? In order to measure precisely the relative degrees of correspondence between various measures and estimates of the abilities of individuals, it is quite evident that something more accurate and exact than mere inspection is necessary. For an explanation of the method by which the exact relationship may be worked out mathematically between the results of a test and the true abilities of the individuals tested, the reader is referred to pages 326–331 in the appendix. The discussion which will be found there of the method of calculating a coefficient of coördination will not be difficult to understand nor will the method be difficult of application for any one who wishes to measure the exact reliability of any of the Mentimeter tests or of any other test. For many purposes such a record as is shown on the preceding page, giving the score of the individual in each test used, will reveal the essential facts regarding the correspondence between test results and demonstrated ability. The reader should be cautious, however, about accepting a conclusion drawn from casual observation of such a table as that shown on the preceding page without checking up the accuracy of this conclusion by actually working out the coefficient of coördination according to the method shown in the appendix. When the reader has tried out, upon a fairly large group of persons of known ability, the Mentimeter tests which seem to him to promise greatest usefulness, and when he has made his calculations and discovered which tests actually do classify his people most accurately, it will then be possible for him to make an intelligent scientific selection of tests for practical use. Let us suppose, for example, that an employer wishes to have a set of tests whereby he may select intelligent sales-girls. By giving the ten or twelve tests which seem most hopeful for the purpose to fifty or sixty saleswomen, who have been in his employ long enough to demonstrate their relative degrees of ability and intelligence, the five or six tests may be chosen whose results show the closest relation to their demonstrated ability for intelligent salesmanship.