In a speech last week, Federal Reserve Board Chair Janet Yellen inadvertently told us why Congress should set a 4 percent unemployment target for the Fed in its conduct of monetary policy, as is proposed in a new bill put forward by Michigan Representative John Conyers. The context was Yellen's dismissal of such a target. Certainly the Federal Reserve Board cannot just pick any number and say it will get the unemployment rate to that level. There are limits posed by the economy that can prevent the Fed from hitting an unemployment rate target despite its best efforts. However, this is also true of the 2.0 percent inflation target that the Fed has chosen for itself as a basis for policy over the last decade. But the fact is that the Fed cannot simply set any inflation rate it likes.