Z-Score at Zero: How I Traded the Setup, and What I'm Watching Next

The z-score hit −2.27 at the March low. The SPX index delivered a 7.3% reversion in 12 days.

The S&P 500 just posted its best weekly gain since November — up 3.6% in five sessions, driven by a ceasefire announcement, a CPI print that came in as feared but no worse, and a breadth recovery from levels not seen since early 2023. For most observers, it was a relief rally. For anyone running a z-score framework, it was something more specific: a mean reversion completing exactly as the statistical signal suggested it would.

The number that mattered was not the index level or the VIX reading. It was the 20-day rolling z-score on the S&P 500, which hit −2.27 on Friday 27 March with the index at 6,368, and extended to −2.11 on Monday 30 March at 6,343. By Friday’s close at 6,817, that z-score had not merely returned to zero — it had overshot to +1.73. In the framework I use, that reading is no longer a neutral signal. It is approaching the fade zone.

This edition is about how that trade looked in practice: the entry logic, the position sizing decisions, and the exit. It also goes into the quantitative mechanics underneath. And it ends with the two forward scenarios I am now tracking, with specific z-score levels that would trigger a new position in either direction.