The fixed-effect approach, while powerful and practical, does not allow one to generalize results to a broader universe of studies and instead requires one to limit the evaluation of study differences to the particular studies at hand. Thus, although inferences cannot be made back to the population of studies of children of alcoholic parents, any systematic study-to-study differences are estimated and accounted for when examining the effects of the predictors of substantive interest. This in turn offers a powerful line of protection for internal validity in that substantively meaningful findings are less likely to be due to differences among contributing studies.
