Define paper trading costs (what “costs” mean)
Paper trading is a practice method where trades are recorded without using real money. Even then, a simulation can reflect “costs” in two main ways: (1) direct costs that would normally be charged per trade, and (2) indirect or execution-related costs that come from how prices are realized when orders are filled.
Direct costs are usually explicit charges (for example, commissions or account/transaction fees). Indirect costs come from the market microstructure and trading process (for example, the difference between quoted prices and the prices at which trades are actually filled). If your simulator only records a theoretical entry price and ignores execution effects, it can understate the real impact of costs.
Direct costs that can affect paper trading results
A paper trading system may model several direct-cost components. Common categories include:
- Commission per trade: Some simulators subtract a commission amount from simulated profits or add it to simulated costs of entry/exit.
- Transaction-related fees: These may include platform or data-related fees, or fees charged for specific order types.
- Financing and carry-like charges (when relevant to the instrument): In many markets, holding positions over time can involve costs. If the paper trading rules include a “hold” mechanism, the simulator may estimate those charges.
Assumption to state when comparing simulations: What does the simulator assume happens at entry and exit? If it uses a fixed commission, a fixed fee, or a fixed financing rate, then your paper results depend strongly on those numeric inputs.
Indirect costs and execution assumptions
Even if direct costs are modeled, results can still change a lot because of execution assumptions. Key drivers include:
- Spread effects: Real trading often involves a bid/ask spread. A paper system may simulate spreads by using different prices for buy versus sell, or it may ignore them.
- Slippage: Slippage is the difference between the intended price and the fill price. A realistic paper model might allow slippage to vary; a simplified one might use the midpoint or last price.
- Order handling: The simulator may assume immediate fills, partial fills, or queued fills depending on market conditions and order type. These choices can change realized costs.
- Latency and timing: If the simulator uses end-of-bar pricing (for example, one price per candle) rather than event-by-event execution, timing-related cost effects can be smoothed away.
A practical way to think about it: paper trading accuracy depends on how faithfully the cost model matches the simulator’s execution logic, not on whether the activity is “real money.”
Evidence and example: how to verify the cost model
Because paper trading is a simulation, verification is mostly about checking the simulator’s documentation and configuration. A useful, self-contained verification approach is:
- Find the simulator’s cost inputs: Look for settings or documentation describing commissions, fees, spreads, and financing/holding rules.
- Check what price series is used: Determine whether the simulator uses bid/ask, mid, last traded price, or end-of-interval values.
- Run a controlled test with explicit assumptions: Use one simple scenario where you know the intended number of units and direction, and then compare the computed simulated P&L change against the simulator’s stated formula.
Example assumption for calculation: Suppose a paper simulator deducts a commission amount per trade and also assumes an effective spread by using a buy price higher than the sell price. If you double the commission setting while keeping all else constant, then the paper results should shift by roughly twice the commission impact (for two affected trade legs). If they do not, your cost model may be applied differently than expected.
Limitations and failure modes (where paper trading can mislead)
Paper trading often fails to reproduce real-world cost effects in at least one of these ways:
- Missing or oversimplified execution: If the simulator assumes perfect or immediate fills at favorable prices, it can understate indirect costs.
- Static cost parameters: Costs like spreads and slippage can vary with volatility and liquidity. A paper model that uses constant assumptions won’t capture that variability.
- Different liquidity conditions: Even without real money, the simulator might not reproduce how quickly orders fill when the market becomes crowded.
- Incomplete market-event timing: Using lower-frequency price updates can remove or distort short-term execution cost.