Semi-monthly 401k contributions auto-buy on a fixed payroll schedule — no choice in timing. Do those buy dates systematically land when the market is locally high? Each contribution's SPY price is compared to the average of the surrounding 14 trading days. One catch: even a random day sits above its local average about 55% of the time, because markets grind up slowly and drop sharply. So contributions are judged against that baseline — not 50%.
Crunching the numbers…
Avg Deviation
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vs window avg
Bought High
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of contributions
Bought Low
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of contributions
Since Jan 2025
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avg deviation
High-Buy Avg
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when high, how high
Low-Buy Avg
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when low, how low
Statistical Sig.
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z-score / p-value
SPY Price + Contribution Dates
Bought high
Bought low
Deviation per Contribution (% vs window avg)
Rolling 12-Month Average Deviation
Is the timing bias getting worse over time?
Distribution of Deviations
Shape of the spread — symmetric or skewed?
Paycheck Timing Breakdown
Mid-month vs end-of-month — one worse than the other?
All contribution dates
| Date | SPY Close | Window Avg | Deviation | Result |
|---|