smoking. At most, the information on smoking in the studies provides some indication of elements of the smoking history, such as number of cigarettes smoked and age of starting to smoke; at a minimum, there is information on whether the participants had ever been regular cigarette smokers. Analyses of the epidemiologic data to characterize the joint effect of smoking and radon-progeny exposure simplify the multiple mechanisms by which the two agents could interact to a mathematical representation or model that typically includes a term for smoking and a term for radon-progeny exposure and a term for their joint action (NRC 1988); alternatively, data have been separately analyzed for ever-smokers and never-smokers (Lubin and others 1994a).

The terminology and methods used to characterize the combined effects of two or more agents have been poorly standardized with substantial blurring of concepts derived from toxicology, biostatistics, and epidemiology (Mauderly 1993; Greenland 1993). The terms "antagonism" and "synergism" refer to combined effects less than or greater than the effect predicted by the sum of the individual effects, respectively. In assessing the presence of synergism or antagonism, a model is assumed to predict the combined effect from the individual effects; lacking sufficient biologic understanding to be certain of the most appropriate model, the choice is often driven by convention or convenience.

The effect of a risk factor for a disease may be considered as acting on an absolute scale or on a relative scale. In the absolute risk model, the risk (r(x)) of disease associated with some factor (x) can be expressed in a simple linear relationship as: