From Tyler Cowan. The prisoners dilemma with real prisoners.
2. From noahopinion on natural experiments.
Briefly, research methods vary on two important dimensions, one we can call internal validity (how sure are we that we know what caused our results?), and the other ecological validity (do our observations relate to the real world?). Only the experimental method can logically show cause-and-effect, so it is highest in internal validity, but the artificial situations created by controlling so many factors make it low in ecological validity (also, experiments can be flawed in many ways, such as poor methods, restriction in the range of observations, confounding factors we didn't think about, etc., which is why replication and attempts to falsify claims are intrinsically important to experimental science). Naturalistic observation is highest in ecological validity, lowest in internal validity. Other methods, such as correlation, ex post facto "experiments" (aka, "natural" experiments), and case studies are in-between on both dimensions.