AMZN: Does the first hour set the tone? Opening-hour vs rest-of-day returns (~3 years)
Does AMZN’s first trading hour reliably “set the tone” for the rest of the day? We looked at intraday minute bars over roughly three years, isolating returns from 09:30–10:30 ET and comparing them to 10:30–close, then testing whether an up or down opening hour predicts the remainder of the session.
The result is straightforward: there’s no meaningful predictive edge. The correlation between opening-hour and rest-of-day returns is essentially zero (Pearson r = -0.028) and the conditional gap is statistically indistinguishable from noise (Welch p = 0.7603). The detailed tables, scatter/box summaries, and robustness checks below show large dispersion and a sign-match rate near coin-flip levels — read on for the full evidence and methodology.
For AMZN over the past ~3 years, does the direction of the first hour's return (9:30–10:30) predict the rest-of-session return (10:30–close) — does the morning really 'set the tone' for the day? Thesis: the opening-hour direction carries essentially no edge for the rest of the day, with if anything a mild afternoon fade, debunking the 'as goes the first hour, so goes the day' adage.
How this was measured
Converted AMZN minute bars to New York time and restricted to regular trading hours (Mon–Fri, 09:30–16:00 ET). Computed per-minute returns within each session (groupby(date).pct_change) to avoid overnight gaps, then compounded via sum of log-returns (log1p/exp1m) across two windows per day: opening hour (09:30–10:30 ET) and rest of session (10:30–close). Aligned days with both windows present and compared rest-of-day returns conditional on the opening-hour direction (up vs down). Reported Pearson correlation and OLS slope between opening-hour and rest-of-day returns, the conditional mean gap with a Welch two-sample t-test, and the share of days where rest-of-day matched the opening-hour sign.
The key numbers
Reading the numbers
Across 752 sessions the average rest-of-day return is essentially zero at 0.000504754, and the mean gap after an up vs down opening is a tiny 0.0002501914 — a negligible difference that does not look like a reliable edge.
The charts
The scatter shows each session's opening-hour return (x, range −0.0427 to 0.0607, mean 0.0002) against the rest-of-day return (y, range −0.061 to 0.0999, mean 0.0005). Points form a diffuse cloud with no clear line; the Pearson r is −0.0283 (p ≈ 0.43895) and the OLS slope is −0.0328, so any negative tilt is vanishingly small. In plain terms, large or small opening moves do not produce a reliable follow-through across the rest of the day.
The box plots compare rest-of-day returns after 'Open up' (n=380, mean 0.00062852, min −0.0448, max 0.0999) versus 'Open down' (n=372, mean 0.00037833, min −0.061, max 0.0414). The distributions overlap heavily and the mean difference (about 0.00025019) is tiny; the Welch two-sample test gives t ≈ 0.3052 with p = 0.7603, so there is no statistically clear difference. Practically, days that start up look very similar to days that start down.
This bar chart collapses direction to sign matches: the rest-of-day has the same sign as the opening-hour on 51.06% of days versus opposite sign on 48.94% (0.5106 vs 0.4894). That near‑50/50 split is essentially a coin flip and does not signal a meaningful predictability. Taken with the scatter and box results, the morning direction does not 'set the tone' in any reliable way.
Rest-of-day summary by opening-hour direction
| bucket | N | mean_rest_ret | std_rest_ret | median_rest_ret |
|---|---|---|---|---|
| Open up | 380 | 0.0006 | 0.0113 | 0.0009 |
| Open down | 372 | 0.0004 | 0.0112 | 0.0004 |
The takeaway
No — the first hour does not meaningfully set the tone for AMZN's rest-of-day return. Over 752 sessions the correlation between the opening-hour return and the 10:30–close return is essentially zero (Pearson r = -0.028) and the OLS slope is slightly negative (slope = -0.0328), while the mean rest-of-day was +0.063% after up opens versus +0.038% after down opens (gap +0.025%). Those differences are tiny and statistically indistinguishable: the Welch test gives p = 0.7603 (about a 76-in-100 chance the observed gap is just noise) and the correlation’s p ≈ 0.439 likewise shows no reliable relationship. The sign-match rate is 51.1%, basically coin-flip territory, and the scatter/box summaries show large intraday dispersion (std ≈ 0.011). Bottom line: there’s no actionable predictive edge in the opening-hour direction — at best a negligible afternoon fade that’s economically small and statistically unconvincing.
The fine print
- Regular-hours only: pre-market and after-hours moves are excluded and can influence opening dynamics.
- Half-days are included, so the 10:30–close window is shorter on some sessions and slightly skews the rest-of-day distribution.
- Three-year sample (N=752) — relationships could change in different volatility or macro regimes.
- Effect sizes are tiny and noisy (mean gap +0.025% vs std ≈ 0.011), so any real effect would be economically negligible.