Cook and Campbell made the crucial distinction between generalizing "to" some population and generalizing "across" subpopulations defined by different levels of some background factor. Lynch has argued that it is almost never possible to generalize "to" meaningful populations except as a snapshot of history, but it is possible to test the degree to which the effect of some cause on some dependent variable generalizes "across" subpopulations that vary in some background factor. That requires a test of whether the treatment effect being investigated is moderated by interactions with one or more background factors.