Execution is how a decision becomes a fill: the order type you send, the price you are willing to accept, and the trade-off between certainty of filling and quality of price.
Target audience: Traders moving a tested system toward live trading who need to model fills realistically.
Execution is how a decision becomes a fill: the order type you send, the price you are willing to accept, and the trade-off between certainty of filling and quality of price.
A backtest assumes a fill; live trading has to earn one. The order type you choose decides whether you pay the spread, whether you fill at all, and how much slippage you take. Two traders with the same signals and different execution can have meaningfully different results, and a backtest that ignores this overstates the edge.
A market order guarantees a fill but not a price: you cross the spread and take whatever liquidity is there, which in fast markets can be several ticks away. A limit order guarantees a price or better but not a fill: you rest in the book and trade only if price comes to you. A stop order becomes a market (or stop-limit) order once a trigger price trades, used to enter on continuation or to exit for a loss. Each one is a different bargain between certainty and price.
Market and triggered stop orders take liquidity: they pay the spread and accept slippage in exchange for certainty, which momentum and breakout systems usually need. Limit orders make liquidity: they can earn the spread but miss fast moves, which suits mean-reversion and patient retest entries. The right choice follows the system's logic. A breakout system that insists on limit fills will skip its best, fastest trades; a mean-reversion system that uses market orders will hand back its edge in spread.
The backtest must execute the way you will. If you will use a stop entry, fill in the test at the trigger plus assumed slippage, not at the signal bar's close. If you will use a limit, fill only when price trades through your level, and drop the trades that never reach it. Execution modelling is not a detail to add later; it is part of the system, and leaving it idealised is how a backtest promises an edge the live account never sees.
Signal: 30-bar-high breakout on a liquid future with a one-tick spread. Stop-entry fill: trigger + 1 tick slippage, captured on every breakout. Limit-entry fill at the level: only the breakouts that retrace fill, removing perhaps 40 percent of trades and changing the expectancy and frequency. Neither is wrong, but they are different systems, and only the one you will actually run should be the one you trust.
Filling in the backtest at the signal price with no spread or slippage.
Using market orders for a mean-reversion system and giving back the edge in spread.
Using limit orders for a breakout system and missing the fastest trades.
Forgetting to drop limit trades that never traded to the limit price.
Treating execution as an afterthought rather than part of the system.
Myth
That you will fill at the price your signal printed.
Reality
No paired reality note provided.
Myth
That a limit order will fill just because price touched it.
Reality
No paired reality note provided.
Myth
That order-type choice is cosmetic rather than part of the edge.
Reality
No paired reality note provided.
For your system, choose an order type per signal and encode the matching fill rule in the backtest.
This lesson is educational content only and is not financial advice. Trading involves substantial risk. A tested process improves decision quality and survivability; it does not predict the market or guarantee any outcome. Trade only with risk you can afford to lose.