Mostly applies to international students hoping to study in the US, or to US students who lack much econ or math background and still want to pursue an econ PhD.
Currently, the major credential (after getting a 168-170 on the quant part of the GRE + very strong math grades (A’s in Real analysis, etc.) and a strong econ course background) needed to get into a top econ PhD program (MIT, Harvard, Stanford, UCB, Princeton, UChicago, Yale, and a few others) is a 2 year predoc at one of the aforementioned schools or a 2 year RA at the Fed.