History of the modern epidemiological concept of confounding.

The epidemiological concept of confounding has had a convoluted history. It was first expressed as an issue of group non-comparability, later as an uncontrolled fallacy, then as a controllable fallacy named confounding, and, more recently, as an issue of group non-comparability in the distribution of potential outcome types. This latest development synthesised the apparent disconnect between phases of the history of confounding. Group non-comparability is the essence of confounding, and the statistical fallacy its consequence. This essay discusses how confounding was perceived in the 18th and 19th centuries, reviews how the concept evolved across the 20th century and finally describes the modern definition of confounding.