You surely can’t contend that only randomized trials can be believed? That would seem to be more a claim of faith than science. Yes, randomization can sometimes be a useful tool. But it can hardly claim to be the only rigorous tool for evaluation. Observational studies will continue to have an important role, especially in the (many) situations in which randomization is not feasible, or it generates results of doubtful (internal or external) validity. This is a well-rehearsed issue; see my paper, “Should the Randomistas Rule?” in the Feb 2009 issue of the Economists’ Voice. Here is the link: http://bit.ly/dGLX9S. Whatever identification strategy is employed, the claims made should follow from the data and assumptions. In this case, two prominent studies, ostensibly using the same data and methods, have come to radically different conclusions on an important issue. That is the mystery. It turns out that one of the studies got it wrong, as explained in Roodman's post cited above. Of course, we all make mistakes. But I only wish the authors had been a bit more humble about their findings, and checked more carefully before going public; it would not seem to have been too hard for them to have seen their errors. That is a lesson for us all, including the randomistas.