On the mathematical theory of errors of judgement with special reference to the personal equation
Karl Pearson · Proceedings of the Royal Society of London · 1901
Abstract In 1896, I, with Dr. Alice Lee and Mr. G. A. Yule, made a series of experiments on the bisection of lines at sight. The object of these experiments was to test a development of the current theory of errors of observation, by which it seemed possible to me a determine the absolute steadiness of judgement of any individual by comparing the relative observations of three (instead of as usual two) observers.