What Is Missing?
In the “methodological Chaps. 15– 21” we have restricted ourselves to the case of only one exposure variable, which, in addition, was supposed to be binary, with the exception of Chap. 21 where we wanted the reader to understand the idea of confounding. However, almost all health outcomes are influenced by several factors acting together; they are “multi-factorial” and “multi-causal”. For example whether a person contracts tuberculosis depends on many of the factors listed in Sects. 1.5 and 7.2 in addition to the intensity and duration of his or her exposition to the pathogen. Genetic factors play a role and so do the environment of the subject, previous infections, her or his life style and her or his social and economic standing. Often some factors “interact” with each other, which means, vaguely speaking, that their joint influence is different from the sum of their influences taken separately. In this case, each factor is called an “effect modifier” of the other ones.