Just when you might have thought every possible metric that might apply to the status of Fed interest rates has been a calculation to incomprehension, Barron's found a new one — the difference between the effective federal fund rate and the yield of the Treasury two-year.

Ever since Federal Reserve chair Jerome Powell confirmed that "time has come for [monetary] policy to adjust," markets have taken this reasonably as something as close to a guarantee the central bank can offer as a promise of rate cuts. But the effective federal funds rate (EFFR) — the average amount banks charge one another for overnight loans without collateral — won't change until it the Federal Open Market Committee makes it.

That means, as of Monday, September 9, the EFFR was 5.33% while the 2-year yield was 3.68%. Subtract the former from the latter and the result is -136 basis points, the most negative figure since the -146 basis point difference in January 2008.