Spread by session vs. the basic “bid-ask spread” idea
Spread by session is a time-windowed way to talk about spread: it focuses on how the bid-ask spread is observed or set during a particular trading session (for example, different market hours where liquidity and quoting behavior change).
The related concept bid-ask spread is simpler: it is the difference between the bid price (the price a market participant can sell at) and the ask price (the price a market participant can buy at) at a particular moment.
Key difference:
- Bid-ask spread answers “What is the spread at this moment?”
- Spread by session answers “What does the spread look like during this specific session window?”
A common source of confusion is that people use the word “spread” without specifying whether they mean a single instant measurement or a time-summarized behavior. Spread by session explicitly pushes you to define which session window you mean.
Spread by session vs. “pip spread” (unit and conversion)
Another related concept is pip spread. A pip is a standardized unit used to express price changes in many forex quotes (for example, based on the decimal precision convention of the pair). Pip spread means the bid-ask spread expressed in pips rather than raw price difference.
Key difference:
- Spread by session is about time context (session windows) and how spread may vary across them.
- Pip spread is about measurement units (converting a spread into pip terms).
These two ideas can be combined in practice, but they are different dimensions:
- You can have a pip spread that is measured at a single time, or averaged/summarized for a session.
- Two quotes can have the same pip spread while occurring in different sessions, or different pip spreads while occurring in the same session, depending on liquidity and quoting.
Spread by session vs. “average spread,” “typical spread,” and “effective spread”
People also compare “spread by session” with general averaging concepts.
- Average spread / typical spread: usually a statistical summary of spreads over a period. The exact meaning depends on the averaging method (for example, simple average of snapshots vs. weighted by time or executed trades).
- Effective spread: a concept often used to describe the spread “experienced” after considering execution details (for example, whether the executed price sits closer to the bid or ask than the quote snapshot did).
Key difference:
- Spread by session primarily establishes the boundary that defines the time window.
- Average/typical spread is about how you summarize within (or across) time.
- Effective spread is about execution experience, not just quotes.
A limitation is that “average,” “typical,” and “effective” can be defined differently by different providers. Even without changing the market, two dashboards can disagree because they compute summaries in different ways.
Spread by session vs. all-in cost (spreads vs commissions vs other fees)
A frequently related idea is all-in cost. In forex trading discussions, “cost” can include:
- the spread (the difference between bid and ask), and
- potential commissions (if charged), plus sometimes
- other trading-related fees.
Key difference:
- Spread by session isolates the spread behavior during defined session windows.
- All-in cost is a broader notion that depends on whether commissions or other charges exist and how they are applied.
Material limitation: even if the spread by session appears stable, all-in cost can still vary when commission structures or fee practices differ. Conversely, all-in cost can appear stable even when spread by session fluctuates, if other components offset it.
What “matters” for spread by session: stable mechanics vs variable conditions
When comparing these concepts, separate stable mechanics from variable conditions:
Stable mechanics (mostly definitional):
- What “bid” and “ask” mean.
- What a pip is for that pair’s quoting convention.
- What “session window” means (start/end times) and how snapshots are grouped.
Variable conditions (can change):
- Liquidity conditions (often different by market hours).
- Volatility (which can widen spreads or change quote depth).
- Order execution details that influence what you actually trade vs what you saw as a quote.
Because the relationship between session and spread is not fixed, spread by session is best treated as a descriptive, conditional observation, not a law.
Evidence or example (with explicit assumptions)
Consider a single currency pair.
Assumption 1: You take quote snapshots every minute. Assumption 2: You define two sessions: Session A (hours when one major market is active) and Session B (other hours). Assumption 3: You compute “spread by session” as the average of the bid-ask spread snapshots within each session.
Under these assumptions:
- If liquidity is higher during Session A, the bid-ask gap might be narrower most of the time, so the session average spread in Session A is lower.
- If liquidity is lower during Session B, the spread might widen more often, so Session B’s session average is higher.
Why this is only an example: the exact numbers would depend on the pair, the quoting behavior, and the averaging method. Also, a one-week sample may not represent other weeks because spreads reflect current market conditions.
Limitations and failure modes to watch
At least one material failure mode is mismatched definitions:
- One source may treat “spread by session” as average of quote snapshots.
- Another source may treat it as average of executed trade spreads.
- Another may use different session boundaries or different time zones.
This can make concepts look inconsistent even when they are internally consistent.
Other limitations:
- Session boundaries can be arbitrary. If you shift the start/end times, the session summary changes.
- Units can confuse comparisons. Comparing price-based spread vs pip-based spread without conversion can mislead.
- Historical summaries do not predict future behavior. Even if a session typically has wider spreads, future market volatility or liquidity events can change the pattern.
How can information about spread by session be verified?
Verification typically comes down to checking what is being measured, and whether the definitions align.
Look for:
- Time window definition: exact session start/end and time zone.
- Measurement method: quote snapshots vs executed trades; averaging or distribution approach.
- Units and conversion: raw price difference vs pip spread.
- Scope: whether any fees/commissions are included or whether it is strictly spread.
A practical next question for readers is: **Does the “spread by session” metric reflect only quotes, or does it reflect executed prices as well?