The comparison of the results for the whole group on the NC and NCWd test given in the bottom line of the table indicates the strength of the interference of the habit of calling words upon the activity of naming colors. The mean time for 100 responses is increased from 63.3 seconds to 110.3 seconds or an increase of 74 percent. (The medians on the two tests are 61.9 and 110.4 seconds respectively.) The standard deviation is increased in approximately the same ratio from 10.8 to 18.8. The coefficient of variability remains the same to the third decimal place ( / m = .171). The difference between means may be better evaluated when expressed in terms of the variability of the group. The difference of 47 seconds is 2.5 standard deviation units in terms of the NCWd test or 4.35 standard deviation units on the NC test. The former shows that 99 percent of the group on theNCWd test was above the mean on the NC test (took more time); and the latter shows that the group as scored on the NC test was well below the mean on the NCWd test. These results are shown graphically in Fig. 1 where histograms and normal curves (obtained by the Gaussian formula) of the two sets of data are superimposed.