of Karl Pearson and others, into the modern science of Biometrics.
When Quetelet tells us, for instance, that the mean stature of the ten-year old boy is 1ôñ273 metres, this implies, according to the law of error, or law of probabilities, that all the individual measurements of ten-year-old boys group themselves in an orderly way, that is to say according to a certain definite law, about this mean value of 1ôñ273. When these individual measurements are grouped and plotted as a curve, so as to show the number of individual cases at each individual length, we obtain a charôÙacôÙterôÙisôÙtic curve of error or curve of frequency; and the ãspreadã of this curve is a measure of the amount of variability in this particular case. A certain mathôÙeôÙmatôÙiôÙcal measure of this ãspread,ã as described in works upon statistics, is called the Index of Variability, or Standard Deviation, and is usually denominated by the letter ü. It is practically equivalent to a determination of the point upon the frequency curve where it