Direct answer
Spread assumptions are the expected effective cost of trading, often represented by a spread and related trading frictions. Costs that can affect those assumptions fall into two broad groups: (1) direct, explicitly charged costs and (2) indirect costs that widen or delay the realized price compared with a simple “quoted spread” assumption.
Mechanics: what “spread assumptions” usually include
In practice, “spread” is the difference between a quoted buy and sell price. Spread assumptions are the inputs you use to estimate how much you will pay (or how costly it is to enter and exit). That estimate can be affected by:
- Explicit transaction costs: items that are charged per trade or per unit traded. Examples include commissions and fixed dealing charges.
- Effective spread vs. quoted spread: even if you assume a certain quoted spread, the price you actually receive can be worse due to execution frictions.
- Financing and carry-related effects: if a position is held, there can be costs or credits tied to overnight or holding periods and the instrument’s currency economics.
- Conversion costs: for multi-currency accounts, currency conversion can add extra friction when funds move between account currency and trade currency.
Key idea: when you model “expected spread,” you are modeling an effective total cost. That effective total cost can be the sum of multiple lines, not only the visible bid–ask difference.
Evidence or example: how costs change the assumed cost
Assume you want to estimate the cost of entering and later exiting a position using spread assumptions.
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If you only model quoted spread, you ignore explicit charges (like commissions) and financing effects. Your estimate can be too low.
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If liquidity changes cause effective spread to widen (for example, during lower liquidity moments), a single “average” spread assumption can fail. Even when the quoted spread looks similar, order execution may occur at less favorable prices.
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If you include holding costs, results depend on your holding period length. Two trades with identical entry/exit quotes can differ materially if one is held longer.
Material limitation: because these costs can be variable with market conditions and holding time, assumptions that work in one period may not translate to another period.
Limitations and risks: where assumptions commonly break
Common failure modes include:
- Assuming stability: treating an average spread from past data as if it will persist.
- Ignoring non-spread frictions: overlooking commissions, conversion effects, or holding costs.
- Mixing time horizons: using short-term spread assumptions for strategies that hold positions over multiple time periods.
- Not separating quoted vs. realized outcomes: assuming the realized execution equals the quoted mid or quoted bid/ask at the moment you modeled.
These risks mean your spread assumptions should be viewed as scenario inputs, not as guaranteed costs.
Verification and next question
You can verify spread-related cost assumptions independently by matching each modeled cost component to contractual or documented terms and then recalculating the effective cost under realistic scenarios.
Practical verification checklist:
- List every cost component you include: quoted spread, explicit fees, financing/holding charges, and any conversion effects.
- Confirm definitions in your execution terms: what the provider counts as charges, when they apply, and how they’re calculated.
- State your assumptions explicitly for any example: the assumed quoted spread, whether commissions are included, and the holding time used for financing-related assumptions.
- Recalculate under multiple scenarios (for example, smaller vs. larger effective spread and different holding durations) to see how sensitive your cost estimate is.
Next question to ask yourself: which cost component in your model is most sensitive to the factor that changes most often for you (execution timing, holding duration, or account/trade currency conversion)?