Pearson believes that “the Gaussian curve will be found to describe effectively the distribution of mental excess and defect” for intermediate ages as measured by Jaederholm's form of the Binet scale. The data on which Pearson places reliance are Jaederholm's results in testing 261 normal children 6-14 years of age in the Stockholm schools and 301 backward children in the special help classes of the same city. The best fit of a normal curve to the data was obtained with a group of 100 8-year-old children, in which case the chances were even that samples from a normal distribution would fit. With his larger normal and backward groups combined in proper proportions in one population the chances were 20 to 1 that such a distribution as was actually found would not fit into the Gaussian distribution. He admits that “this is not a very good result,” although it is better than when the Gaussian curve is fitted to either the normal or the backward group alone. In a subsequent paper he gives each child a score relative to the standard deviation of the normal child _of his own age_, a method comparable to his treatment of Norsworthy's data. He then finds that “10% to 20% or those from 4 to 4.5 years and beyond of mental defect could not be matched at all from 27,000 children” (_164_, p. 46). In each case the distributions actually found were skewed somewhat toward deficiency. Furthermore, when he suggests that -4 S. D. may be used as a borderline for tested deficiency, he recognized that the mental ability of children is skewed so far as the empirical data are concerned. With a normal distribution there would not be two children in 100,000 who would fall below this borderline. Nevertheless, the normal curve serves for most practical purposes to describe the middle ranges of ability. Pearson thinks that the skewed distributions of his data may possibly be explained by the drawing off of older children of better ability to the “Vorgymnasium,” or to the higher-grade schools, by the incompleteness of the higher age testing, or by the “possibility of the existence of a really anomalous group of mental defectives, who, while continuously graded _inter se_, and continuously graded with the normal population as far as intelligence tests indicate, are really heterogeneous in origin, and differentiated from the remainder of the mentally defective population” (_164_, p. 34). The last hypothesis, of course, supposes that mental ability is skewed and suggests the cause. He supplements this explanation by stating that the heterogeneous cause of the “social inefficiency” of the deficients may not be connected directly with the intellect but affect rather the conative side of the mind. A skewed distribution under biological principles of interpretation supposes a single cause or group of causes especially affecting a portion of the population.