S&P 500 daily close (FRED series SP500) and WTI crude oil spot price (FRED series DCOILWTICO), https://fred.stlouisfed.org/series/SP500 and https://fred.stlouisfed.org/series/DCOILWTICO, 4 January 2021 – 22 September 2026 (post-dates the April 2020 negative-price anomaly), downloaded 27 Sep 2026.