ResearchQuantitative Methods

QUANTITATIVE METHODS

Why Execution Assumptions Distort Backtests

Quantifying the gap between simulated and realized fill quality

Blackmark Dominion Research Inc. · February 24, 2026 · 14 min read

Abstract

Every backtest makes assumptions about how orders are filled. The most common assumption — filling at the closing price or mid-quote at signal generation time — is also the most dangerous. It systematically overstates achievable returns and understates risk.

Measuring the Gap

We construct a dataset of 2,400+ strategy-months across equities, commodity futures, and G10 FX, comparing backtest P&L under standard assumptions against realized execution logs.

The gap is substantial:

  • Equity momentum strategies: 60–120 bps annual overstatement
  • Commodity trend-following: 40–90 bps annual overstatement
  • FX carry strategies: 80–180 bps annual overstatement
  • Recommendations

  • Use conservative fill assumptions in all backtests
  • Model market impact as a function of position size and ADV
  • Apply regime-dependent slippage estimates
  • Validate execution assumptions against live trading data quarterly
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