The Correlation Coefficient
Ronald Aylmer Fisher, Frank Yates · 1990
Abstract 30. No quantity has been more characteristic of biometrical work than the correlation coefficient, and no method has been applied to such various data as the method of correlation. Observational data in particular, in cases where we can observe the occurrence of various possible contributory causes of a phenomenon, but cannot control them, have been given by its means an altogether new importance. In experimental work proper its position is much less central; it will be found useful in the exploratory stages of an inquiry, as when two factors which had been thought independent appear to be associated in their occurrence ; but it is seldom, with controlled experimental conditions, that it is desired to express our conclusion in the form of a correlation coefficient.