Two reasons are largely responsible for these low coefficients. The first is the fact that the measures correlated have usually been initial trials, or at most averages of a very few trials. This means great individual variability and considerable consequent unreliability of the data. A more important factor, perhaps, is the fact that these preliminary trials do not necessarily represent the final capacities of the individuals. They are determined by a host of incidental or accidental influences and reveal only momentary ability, not ultimate capacity. There is every reason for expecting to find positive correlation of "desirable" traits, and we may well expect to find this increasingly true the more our measures test the final limits of capacity in the various tests. In other words, the only real correction for unreliable measures is to be made by continuing the test until the individual has reached the limit of practice in it.
Only occasional attempts have been made to determine the influence of practice on the correlation of abilities, and those that have been reported have been based on so few practice trials that no review of them need be given. In the present chapter I shall present the results of an experiment in which a group of observers were repeatedly tested until in each test a practice limit was approximated, a limit which, in most cases, one hundred further trials failed to improve. The results have a real interest for vocational psychology.
The experiment consisted in putting each of thirteen individuals through 205 repetitions of seven different mental tests. The trials were controlled as thoroughly as possible with respect to such factors as interim occupation, exercise, food, rotation of tests, temperature, illumination, and incentive and interest. The subjects, four women and nine men, ranging from eighteen to thirty-nine years in age, were mature, zealous, and faithful. Competition was stimulated by the award of desirable prizes, and each worker received a daily wage. Records were announced to the subjects only after each thirty-five trials. So far as previous practice in these particular tests is concerned, all the subjects were naïve. Five trials were made daily, these trials being distributed through the day at about two-hour intervals. The tests themselves occupied about forty minutes at each sitting.
The tests used were the following familiar laboratory forms:
1. Adding. Adding seventeen mentally to each of fifty two-place numbers and reciting aloud the correct answer. Order of numbers random at each trial. Record with stop watch, time required for perfect score.
2. Naming Opposites. Correctly naming opposites of each of fifty adjectives which occurred each time in random order. Record, time required for a perfect score.
3. Color Naming. The Columbia laboratory form of this test, with ten repetitions of each of twelve colors. Position of card changed at each trial. Record, time required for perfect score.
4. Discrimination Reaction. Discriminating between red and blue, and reacting correctly with appropriate hand. Record, average time, in sigma, and number of false reactions.
5. Cancellation. Crossing out digits from the Woodworth-Wells form of this test. Record, time required for 75 correct cancellations of equally difficult digits.
6. Coördination. The familiar three-hole test, for accuracy of aim. Record, time required for one hundred correct strokes.