{"content":[{"text":"THE CAUSES OF HUMAN VARIABILITY.1 \n By R. A. FISHER. \n THE great service which the modern development of statistics \n has rendered to eugenics is that it supplies a definite method \n of measuring and analysing variability. Only so can the true \n causes of variability be ascertained and the factors which are \n of no effect, the false claimants to importance in this regard, be \n excluded from consideration. \n The practical method of measuring the variability of 'a \n population is extremely simple, although explanations of it are \n apt to appear complicated at a first reading. If a measurement \n of stature, for example, is made upon a sufficiently large sample- \n I,OOO to 10,000, let us say, of a population-the measurements \n are usually found to be grouped symmetrically about a mean \n value, the average stature of the sample. The deviations from this \n average follow very closely in their distribution what is known \n as the law of errors, that is to say the frequencies with which \n deviations of different magnitudes occur are related in the same \n way as the frequencies of errors of observation. Consequently \n the amount of variability may be measured, as errors of observation \n are habitually measured, by the mean of the squares of the \n deviations of different individuals from the mean, this mean \n square deviation being strictly comparable to the mean square \n error used in astronomy and in all physical science. \n Thus from I,0521 measurements of stature, Karl Pearson \n and Alice Lee found a mean stature 62@48' inches, and a mean \n square deviation 5*712 square inches. This mean square deviation \n I term the variance, and use it as the measure of variability, by \n reason of this important property, namely, that two independent \n causes of variability acting together produce a variance which is \n the sum of the variances produced by either separately. The \n square root of the variance has long been known as the standard \n deviation. The variance, then, is a measurable feature of the \n I Trans. Roy. Soc. Edin. Vol. LII., Part II. (No. 13), pp. 399-434. The Corre- \n lation between Relations on the Supposition of Mendelian Inheritance. \n 2These measurements are those of the mothers, of whom the number measured is \n given.","bbox":{"left":0.11108,"top":0.139322,"width":0.723713,"height":0.744947}}],"images_bbox":[],"images_bbox_not_text_overlap":[]}