By the end of the year, President Obama will likely make one of the most important economic decisions of his presidency: appointing a new Federal Reserve chairman. The two leading candidates, according to reports, are Janet Yellen, the Fed’s vice chairman, and Larry Summers, a former member of the president’s economic team and Secretary of the Treasury under the Clinton Administration. Both have impressive economic credentials but differ in background and reputation, raising the question of what, exactly, makes a great Fed chair.