Causal Inferences, Closed Populations, and Measures of Association

Two of the most important traditions of quantitative research in sociology and social psychology are those of survey research and laboratory or field experiments. In the former, the explicit objective is usually that of generalizing to some specific population, whereas in the latter it is more often that of stating relationships among variables. These two objectives are not thought to be incompatible in any fundamental sense, but nevertheless we lack a clear understanding of their interrelationship.One of the most frequent objections to laboratory experiments turns on the question of generalizability, or what Campbell and Stanley refer to as “external validity.” In essence, this question seems to reduce to at least two related problems: (1) that of representativeness or typicality, and (2) the possibility of interaction effects that vary with experimental conditions. In the first case, the concern would seem to be with central tendency and dispersion of single variables, that is, whether the means and standard deviations of variables in the experimental situation are sufficiently close to those of some larger population. The second involves the question of possible disturbing influences introduced into the experimental setting that produce non-additive effects when combined with either the experimental variable or the premeasurement. These same variables may of course be operative in larger populations. But presumably they take on different numerical values, with the result that one would infer different relationships between major independent and dependent variables in the two kinds of research settings.