In order to assist readers who have no statistical training in the evaluation for their special purposes of any particular Mentimeter test, a few pages will be devoted to an elementary statement of how to try out scientifically the relationship between a test, on the one hand, and demonstrated ability in any special line of endeavour, on the other. It may be stated here again that not all traits of mind are important in every task that must be done in life. Some positions require only a little intellectual ability while others require a great deal, and some tasks require very great development of a few traits which may be very little called for in other equally important tasks. The authors have used their best judgment as to which tests will probably select the type of persons needed in a certain type of position, but the judgments of other equally experienced men would be just as good. The final proof of reliability in a test can come only by actual trial of that test upon men of various degrees of demonstrated ability in the trade or profession concerned. What follows is a statement of how to measure this correspondence between demonstrated degree of success and score in a test, or between the scores of the same persons in two or more different tests. No measure of relationship between success in life and success in a test can be any more accurate than the original measures of success from which the calculation is made. If the measures of success in life are unreliable, then the measure of their relationship to success in a test will be even more unreliable. The more definite and certain one can be of his measures of success, the more reliable will his measure of relationship be. In productive labour, especially where payment is based upon the number of standard articles produced in a day, or upon the number of standard operations performed in a given time, the records of actual performance are probably the best measures of success available as a standard against which to judge the reliability of a test. The record for one day or for one week would be less reliable usually than the record for a month or a longer period. In many business organizations and industries there is no such satisfactory standard of success as individual production records, and in such cases it is necessary to make use of the judgments of foremen, supervisors, or superintendents. These are far less satisfactory records of efficiency and are subject to gross errors and prejudices, but they are the only available measures of many workers. If the rating as to ability is the consensus of the judgments of two or more supervisors, each making his rating without any reference to that made by any other person, the result is much more reliable than the rating of any single supervisor would be.