The variation in age norms with different tests is shown graphically in Figures 6, 7 and 8. In order that the various tests may be plotted on the same scale, so as to compare changes in development for the different tested processes, I have used the average increase in ability from 8 to 9 years of age for each test as a common measure and arbitrarily plotted the slant of the curve between these ages at 45 degrees. The increase from 8 to 9 is represented by 10 units on the objective scale to the left of the graphs. On this basis it is possible roughly to compare changes in the absolute annual increase at different ages for the same test and for different tests. It assumes that the units in which each test is scored are equivalent for that test. An average difference between the basal ages or between any two ages cannot be assumed to be accompanied by the same distribution of increases. Moreover, the 8-year norm is at different distances from zero for the different tests so that the relative increase from 8 to 9 cannot be regarded alike for the different tests. The method, however, is sufficiently accurate for illustrating the very different forms of the developmental curves which might be expected if they were measured by absolute increases from year to year. Even the variation in the slant of the lines at the different ages gives a graphic picture which will assist in interpreting the significance of average curves of general ability. As the curves stand, they show the norms for each age for any test, as if placed on its own objective scale, and the various objective scales have been harmonized on the assumption that the norms at 8 and 9 years are accurate. We thus have a simple representation of the absolute changes in the abilities tested from age to age by the same tests relative to a single objective scale. It will not give a seriously erroneous picture for any tested ability so long as the units in which the particular test is scored may be presumed to be objectively equal.
The tests on which Figures 6, 7, and 8 were based included practically all which were reported in the researches used. They were as follows: Norsworthy ([159]), perception of 100-gram weight, cancelling A's (boys), ideas remembered from four simple sentences, memory of related and of unrelated words, part-wholes, genus-species, opposites and reverse of opposites given the next day, “a-t” test. J. Allen Gilbert ([108]), taps in 5 seconds, fatigue in tapping, visual reaction time, color-discrimination reaction time, reproduction of 2-second interval. Smedley (51, No. 3), strength of right-hand grip (boys), taps in 30 seconds (boys), ergograph; visual, auditory, audio-visual, and audio-visual-articulatory memory for digits. W. H. Pyle, Standards of Mental Efficiency (J. of Educ. Psychol., 1913, IV., 61-70), uncontrolled association, opposites, part-wholes, genus-species, digit-symbol and symbol-digit substitution, memory for concrete and for abstract words, memory of Marble Statue selection, (only boys' norms used for each). Pyle and Anderson combined by Whipple ([220]) two word-building tests (boys). Anderson as given by Whipple memory for letter squares. D. F. Carpenter, Mental Age Tests (J. of Educ. Psychol., 1913, IV., 538-544), substitution of colors in forms and of numbers in forms, perception time in marking A's, concentration, i. e., difference in time of last test under distraction, memory of pictures of objects, all tests devised by Carrie R. Squire. Stenquist ([54]), construction test. Sylvester ([191]), form-board test.
Fig. 6. Tests of the Development of Memory Processes. Medians at Each Age of the Central Tendencies of the Tests.
Fig. 7. Different Types of Development. Medians at Each Age of the Central Tendencies of the Tests.
Fig. 8. Forty Curves of Development. Distribution at Each Age of the Central Tendencies of the Tests.
In Fig. 6 curves A and B are Smedley's tests; curve C includes in addition Norsworthy's unrelated words, Pyle's memory for concrete and abstract terms, Anderson's letter-squares, Carpenter's memory for pictures, and Gilbert's for the time interval; curve E includes Pyle's two and Carpenter's two substitution tests; curve F includes Pyle's Marble Statue and Norsworthy's memory for related words and for sentences; curve S is Norsworthy's; curve D is the combination of these 17 tests.
In Fig. 7 curve H includes Gilbert's visual reaction time, Norsworthy's A and a-t tests, Carpenter's two A tests; curve I includes Gilbert's and Smedley's tapping tests; curve J is the median of the central tendencies of all 40 tests; curve K includes Norsworthy's two opposites and her part-whole and genus-species tests, the Pyle opposites, genus-species and part-whole tests; curve L is the same as D, curve M includes Smedley's strength of grip and ergograph tests and Gilbert's fatigue of tapping; curve N includes Pyle and Anderson's word building tests and Pyle's uncontrolled word association test.