I have no wish, nor the skill, to embark upon a philosophical discussion of the meaning of'causation'. The'cause'of illness may be immediate and direct, it may be remote and indirect underlying the observed association. But with the aims of occupational, and alnost synonymously preventive, medicine in mind the decisive question is whether the frequency of the un-desirable eventB will beinfluenced by a change in the environmental feature A. How such a change exerts thatinfluence may call for a great deal of research. However, before deducing'causation'and taking action we shall not invariably have to sit around awaiting the results of that research. The whole chain may have to be unravelled or a few links may suffice. It will depend upon circumstances.

Disregarding then any such problem in semantics we have this situation. Our observations reveal an association between two variables, perfectly clear-cut and beyond what we would care to attribute to the play of chance. What aspects of that association should we especially consider before deciding that the most likely interpretation of it is causation?