Last update:
07-Jul-2004

The term “interaction” is used in statistics, epidemiology and pharmacology. In statistics, “interaction” is used to refer to departure from the underlying form of a statistical model (additive or multiplicative). In epidemiology the expression “effect measure modification” is usually used instead of the term “statistical interaction”. Εffect measure modification is said to occur when the magnitude of the association between one variable and another differs according to the level of a third variable. For example, obesity increases the risk for breast cancer in postmenopausal but not premenopausal women. Thus menopausal status is a modifier of the effect of obesity on breast cancer. A modifier is a characteristic of the individual which modifies the effect of an exposure on the incidence of disease. In this way the modifier is the characteristic that changes the relation between exposure and disease. The presence or absence of effect measure modification depends on the arbitrary choice of an effect measure. The effect measures usually used are the risk ratio (or incidence density ratio) and the risk difference (or incidence density difference). When the use of one effect measure (for example risk ratio) does not indicate modification, then the use of another effect measure (for example risk difference) may demonstrate modification. Thus blanket statements about the presence or absence of effect measure modification cannot be made based on a single effect measure. Apart from interaction with the meaning described above, there is also biological interaction, which refers to mechanical interaction which may or may not occur. Biological interaction can be described more accurately as biological co-action. More specifically, under the sufficient cause model, biologic co-action is defined as the participation of two or more component causes in the same sufficient cause model to produce a disease with a specific mechanism. Biological co-action is also defined as synergistic coaction or simply as synergism, when the development of the disease requires the synergistic participation of two causal factors. Conversely, failure of a disease to occur because two factors are both present is defined as antagonistic co-action between the two causal factors, or antagonism.

Key words: Biological co-action, Biological interaction, Effect measure modification, Modifier, Statistical interaction.