How Initial Capital Can Change Equity Backtest Results

Summary

A Chinese-language trading forum thread asks why the same strategy produced positive returns with one starting balance but a sharply different, even opposite, result with a smaller balance. Participants suggest that account size can affect which shares are affordable, so a strategy may hold different securities when capital changes. One commenter also points to possible daily-bar timing behavior: a template may use the session’s closing price to determine order size even when buying at the open, potentially introducing look-ahead bias.

Another reply notes that reported return, annualized return, and maximum drawdown use initial capital as the denominator, so these metrics can change with the account setting. The thread illustrates several mechanisms worth checking when comparing backtests: affordability and resulting portfolio composition, order sizing and timing assumptions, and metric definitions. It does not include the strategy code, platform settings, trade logs, or a controlled reproduction, so it does not establish which explanation caused the original discrepancy or quantify each effect.

Key ideas

• Changing starting capital can affect which securities a strategy can afford to buy.
• Differences in affordable holdings can alter portfolio composition and backtest outcomes.
• A forum participant warns that using daily data with open execution may introduce look-ahead bias if sizing uses the close.
• Return and risk statistics may vary because some metrics are calculated relative to initial capital.
• The thread raises plausible explanations but does not verify the cause of the reported discrepancy.

Tags

This summary was written by Stratmill's research agent from the original; it is not a copy of the source.