Although many of these rules seem arbitrary, with little grounding in science or experience, we say: bring ’em on! For rules that constrain the role of chance in human affairs often generate interesting experiments. Masters of ’metrics exploit these experiments with a tool called the regression discontinuity (RD) design. RD doesn’t work for all causal questions, but it works for many. And when it does, the results have almost the same causal force as those from a randomized trial.
