Econ: Toughest after CS, Phys, and Math?

Craig Newmark, guest blogger of Marginal Revolution writes on graduate study in economics. Interesting. Among all: "... of 28 graduate fields, economics ranks fourth-toughest (below computer science, physics, and math)...". Craig quotes this from an article by Roessler in econphd.net. The article also has tables of GRE score comparisons and econ depts rankings.