Spread definition: a measurable concept
“Spread definition” is measurable when you turn it into explicit fields and a repeatable calculation. In practice, spread refers to the difference between a buy price (ask) and a sell price (bid) quoted by a provider at a specific moment.
To measure it, you need:
- A quote source: the provider’s displayed bid/ask (or an API feed).
- A timestamp rule: when to record the bid and ask (and with what granularity).
- A calculation rule: spread = ask − bid, expressed in points, pips, or another agreed unit.
Mechanism: what you record and how you compute it
Start with a definition you can reproduce. For example, if your quote system shows bid and ask for the same instrument, you can compute:
- Raw spread (price difference): ask(t) − bid(t)
- Spread in points/pips: convert the price difference using the instrument’s pip/point convention.
To “measure spread definition” consistently across time or across providers, you also need to state assumptions:
- Same instrument mapping: confirm the underlying symbol, contract size, and quote convention match.
- Same quote type: distinguish displayed quotes from executable prices if your platform differentiates them.
- Same timestamp handling: decide whether you use a single instant (same t for bid and ask) or the nearest available time if quotes arrive asynchronously.
A useful practice is to store the bid, ask, computed spread, and the timestamp together. This creates a dataset where another person can re-run the same calculation.
Evidence and example: a comparison method you can verify
A simple measurement workflow for a single instrument over time:
- Choose a fixed set of time points (for example, every N seconds) and record bid(t) and ask(t).
- Compute spread(t) = ask(t) − bid(t).
- Summarize with statistics such as the median spread and a chosen percentile.
For cross-provider comparison, repeat the same workflow under clear constraints:
- Use the same time points.
- Use the providers’ respective quote sources, but apply the same spread formula.
- Report both the computed spread and the exact rules (timestamp sampling interval, unit conversion).
This approach does not assume the market is “stable”; it measures what each provider defines and displays at the times you select.
Limitations and failure modes
The biggest limitation is that spread depends on conditions that change quickly. Even with identical calculation rules, results can differ due to:
- Variable market liquidity: rapid price changes can widen spreads.
- Execution versus display: the price you can actually transact at may not match displayed quotes.
- Asynchronous timestamps: if bid and ask updates are not captured at the same instant, the measured spread can be distorted.
- Conversion errors: incorrect pip/point conversion can produce misleading comparisons.
Another failure mode is assuming historical relationships carry forward. Historical spreads reflect past conditions and may not represent future behavior.
Verification and next question
Independent verification is possible if you publish the measurement protocol: quote source, timestamp rule, spread formula, and unit conversion. The next question to clarify is whether your “spread definition” should refer to displayed quotes or executable transaction prices, because those can measure different realities. If you want, share your intended quote source and timestamp method, and you can validate the measurement fields and assumptions before collecting data.