The Work of Thomas Cook and Donald T. Campbell Internal Validity addresses correct causal statements.
Experimenters can determine the exposure of the dependent variable.
Generalizability The best way to enhance external validity is to take probability samples from the population to which generalization is desirable. Another method is to sample across several groups and determine relationships among heterogeneous populations. This produces a quasi-generalizability. The idea is that multiple categories and limited conclusions are superior to indefensible generalizations on 'make-do' probability samples. It is better to sample LA, Bakersfield, and Sacramento cops and generalize results across these cities than to do a flawed probability sample of all cops in California and try to generalize to.