In practice, the effective FF tends to occasionally diverge from this corridor, and when it does, it prompts fears that the Fed is losing control over the most important instrument available to it: the price of money, which is set via the fed funds rate. And ever since March 20, this fear is front and center because as shown in the chart below, starting on March 20, the effective Fed Funds rate rose above the IOER first by just 1 basis point, and then, last Friday spiked as much as 4 bps above IOER.
