Contributory cause: unnecessary and insufficient.

Contributory cause is a clinically useful concept of causation. It requires demonstration that (1) the presumed cause precedes the effect and (2) altering the cause alters the effect. It does not require that all those who possess the contributory cause experience the effect. It does not require that all those who are free of the contributory cause be free of the effect. In other words, a clinical cause may be neither necessary or sufficient but it must be contributory.