SPDR S&P 500 ETF (SPY): ETF Research Reports However, as esteemed Wharton School Professor Jeremy Siegel pointed out this morning on CNBC, core CPI is historically among the last metrics to respond to anti-inflation measures. Headline CPI was down for the third-straight month last week, after all, demonstrating that the Fed’s higher interest rates are indeed having the desired effect, if not quite on the schedule most investors would like to see.