Biological variation and quantification of health: the emergence of the concept of normality.

Historical research on the concept of normality, its roots and its development show that this concept has its sources in very different areas of scientific and medical thinking. Of great importance were: (i) a new theory of disease arising early in the 19th century supposing a continuous change from the healthy to the diseased state; (ii) the examination of variation within and between species of plants and animals; (iii) the clinical theory of constitution developed to describe the wholeness of the individual determined both by genetic factors and the influence of the environment; (iv) the development of mathematical and statistical tools starting with the adaptation of Bernoulli's "law of the great numbers" and Gauss' and Legendre's "law of errors" to biological measurements by the Belgian astronomer Quetelet. At the end of the First World War the concept of normality was first discussed. An idealistic "value norm" was set against a statistical "frequency norm". Between 1920 and 1930 the principles of our present concept were accepted and the mathematical tools developed. It took several decades to introduce this concept into practical medicine finally being recommended by the IFCC.