What “spread by session” means
Spread by session is a way to describe the effective difference between buy and sell prices within a defined trading time window (a “session”). In practice, people often look at whether the typical difference is larger or smaller during specific hours, such as when markets are more liquid or when spreads widen.
When discussing “what costs affect it,” it helps to separate two ideas:
- Market spread: the price gap available for buying versus selling.
- Trading costs: charges and frictions that can make the effective cost higher (even if the displayed quote spread seems similar).
Because cost components can be added by the provider or incurred during execution, they can change the effective spread you experience from session to session.
Direct costs that can change session-to-session
Direct costs are charges that appear explicitly in your trading economics. Common examples include:
- Commission or commission-based pricing: If commission is charged per order or per trade, total trading cost can rise or fall with activity volume during different sessions. Even if the quote spread stays similar, commission can change the effective difference you pay.
- Financing or carry-related charges: Positions held over time can incur financing-related costs depending on the instrument and provider rules. Different sessions can matter because they determine when positions are opened and may affect which parts of a holding period trigger those charges.
- Fees for specific execution types: Some providers may apply different fee structures depending on order routing or execution approach. If those choices are used more frequently in certain sessions, effective cost can differ.
Assumption for examples: if you compare session A and session B using the same instrument and roughly similar trade size, then commission and financing charges can be treated as additive to any quote-based spread effect. If trade timing differs materially, that assumption may not hold.
Indirect costs and frictions that can shift the effective spread
Even when the quoted spread is unchanged, indirect costs can alter what you effectively pay or receive.
- Slippage from execution: Orders may execute at worse prices than the last quoted or intended level, especially when liquidity is thinner. Session boundaries often align with liquidity changes, so slippage risk can be higher in less active hours.
- Market impact (your trading can move prices): Larger orders, higher urgency, or lower depth can lead to worse execution. This can vary by session because depth and order-book resilience vary with time.
- Quote/price timing effects: What you observe as spread depends on measurement timing (snapshot versus averaged over time). A session with fast-moving quotes can produce different realized outcomes than a session with stable quoting.
A material limitation: slippage and market impact are not solely “costs” charged by a provider; they are execution outcomes tied to market conditions and order behavior. Therefore, cost attribution can be uncertain without detailed execution logs.
Limitations and failure modes to watch
- Confusing “quoted spread” with “effective trading cost”: A session can show a narrow quoted spread but still produce high effective cost due to commissions, financing charges, or execution quality.
- Changing assumptions across sessions: Trade size, order type, and holding time can change between sessions. If these differ, comparing spreads across sessions can lead to incorrect conclusions.
- Historical patterns do not guarantee future behavior: Past session effects may not repeat because liquidity and pricing dynamics change with market conditions.
These limitations mean that even a careful analysis can fail if you do not maintain consistent definitions and measurement windows.
How to verify the relevant facts independently
To verify what costs affect your spread-by-session view, use a checklist focused on definitions and data consistency:
- Define the measurement: Decide what “spread” means in your comparison (quoted bid-ask gap, averaged value, or realized execution cost).
- Collect provider documentation: Look for sections that describe commission, financing/carry rules, and any relevant fee schedules. Use those rules to list cost components that apply to your instrument.
- Use execution records: Compare the difference between intended and executed prices, plus any explicit fees shown in trade statements.
- Hold variables constant where possible: Compare sessions using the same instrument, similar order size, similar order types, and a consistent time window.