Prior to the Lehman failure in 2008, the Federal Reserve controlled the federal funds rate through open market operations that added to or subtracted from the excess reserves that banks held at the Fed. Because excess reserves typically were only a few billion dollars, the funds rate was very sensitive to small changes in the quantity of reserves in the system.

The Fed’s response to Lehman and its aftermath included large-scale asset purchases that led to a thousand-fold increase in excess reserves. Consequently, since 2008, small open-market operations of a few billion dollars no longer alter the federal funds rate. Instead, the Fed introduced administered rates to change its policy stance. The most important of these—the interest rate that the Fed now pays on excess reserves (IOER)—sets a floor below which banks will not lend to other counterparties (since an overnight loan to the Fed is the safest rate available).

Until very recently, the Fed’s ability to control the federal funds rate seemed well in hand….