This Mentimeter is composed of two parts, one of them measuring the difficulty of the addition problems which a candidate can solve, and being therefore a fairly good test of intelligence, the other measuring the speed at which one can add fairly easy problems and therefore having less relations to intellectual ability. Teachers in school furnish the children with certain information and knowledge or guide their pupils to the sources from which this information and knowledge may be obtained. In Addition, this information takes the form of “2 and 2 make 4,” “9 and 5 make 14” and such sums. The instruction also involves such things as how to carry in addition, how to add when a decimal point enters into the calculation, and all such problems. The teacher also undertakes a somewhat different task, which is the review and drilling of the pupils in order to make the combinations which have been explained automatic in the minds of the children. It is not sufficient for John to know that “3 and 5 are 8,” but he must be able to write or say “8” without any hesitation whatever when 3 and 5 are to be added. In order to test the extent to which teachers have explained addition, one would measure the pupils with a test which increased in difficulty from the first problems to the last, undertaking to determine how difficult a problem can be solved. In order to test the rapidity with which the pupils have learned in their drill exercises to make the different combinations, one would measure the speed with which they can make the simple combinations. The first addition test which is given on page 175 measures the extent of the information which the candidate has gained, while the second measures the speed which the candidate has developed by drilling upon addition problems. Teachers in the public schools will undoubtedly wish to measure both the speed with which their pupils do simple problems as a result of drill, and the difficulty of the problems which the child can solve correctly as a result of instruction. Of course the test for difficulty of problems solved is not absolutely independent of the effect of drill, although it is relatively much less dependent upon drill than is the other test. In industrial organizations the first test given will probably be found most useful, while in social groups the greatest amount of entertainment will probably be obtained from the second test. The reliability of the results from the first test is very much higher than the reliability of the results from the second test, for in the second case a difference of one second in time allowed may make as much as one or two points difference in the score obtained.