Backtesting correctly means simulating decisions using only the information, costs, liquidity, and constraints that would have existed at the time. Anything else risks turning historical data into a misleading sales pitch.
Protect against timeline errors
Signals must be generated only from data available before the trade decision. Revised fundamentals, adjusted prices, index membership, and future corporate actions can quietly leak future information into a backtest.
Use realistic order and cost assumptions
The backtest should include commissions, spread, slippage, market impact, order delay, liquidity limits, and partial fills where relevant. Strategies should be tested under conservative costs, not only ideal execution.
Validate across regimes and parameter choices
A robust strategy should not depend on one perfect parameter set or one lucky market window. Out-of-sample tests, walk-forward validation, stress periods, and sensitivity analysis help identify fragile results.
Strategic takeaway
A correct backtest is a risk control. It does not prove future profits, but it can prevent obvious research errors from reaching live capital.
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