About a decade ago, I wrote a paper on monetary policy in the 1990s (published in this book ). I estimated the following simple formula for setting the federal funds rate:Federal funds rate = 8.5 + 1.4 (Core inflation - Unemployment).Here "core inflation" is the CPI inflation rate over the previous 12 months excluding food and energy, and "unemployment" is the seasonally-adjusted unemployment rate. The parameters in this formula were chosen to offer the best fit for data from the 1990s. You can think of this equation as a version of a Taylor rule. Eddy Elfenbein has recently replotted this equation. Here it is:The interest rate recommended by the equation is the blue line, and the actual rate from the Fed is the red line.Not surprisingly, the rule recommended a deeply negative federal funds rate during the recent severe recession. Of course, that is impossible, which is why the Fed took various extraordinary steps to get the economy going. But note that the rule is now moving back toward zero. As Eddy points out, "At the current inflation rate, the unemployment rate needs to drop to 8.3% from the current 8.5% for the model to signal positive rates. We’re getting close."