Execution realism
Transaction Costs: Where Paper Alpha Goes to Die
Gross performance describes a signal; net performance determines whether the implementation still has an economic claim.
Working definition
A transaction-cost model converts intended position changes into estimated net returns using fees and the market frictions implied by the strategy's instruments, timing, and scale.
01
Costs follow the trading mechanism
A daily cross-sectional rebalance, an intraday market-making rule, an options spread, and a short equity book need different cost components. Commission alone is rarely the dominant uncertainty. Spread, market impact, borrow, financing, exchange fees, roll costs, and rejected or partial executions can determine whether gross alpha is realizable.
Turnover should be derived from the actual position path and reconciled with the ledger. A cost rate applied to an unrelated trade count can look precise while missing the quantity that generates cost.
02
Capacity is a scenario, not a footnote
Impact is nonlinear as participation grows. A credible report shows performance at the proposed capital level and under larger participation or thinner liquidity. The capacity estimate should use information available at the simulated time and distinguish average volume from volume accessible to the strategy's execution window.
- Reconcile turnover, position changes, and charged costs.
- Separate explicit fees from spread and market impact.
- Include borrow and financing where the positions require them.
- Show sensitivity across capital and cost assumptions.
03
Look for margin, not a single breakeven point
A strategy that becomes unattractive after a small, plausible cost increase has weak implementation robustness. Validation should identify the cost level that removes the edge and compare it with both the central estimate and an adverse scenario.
Practical takeaways
- Model every friction implied by the instrument and holding process.
- Charge costs from the actual position changes.
- Evaluate capacity at explicit capital and participation levels.
- Report the margin between expected and breakeven costs.
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