How can Buy Limit be measured?

Mechanism and definition of a measurable Buy Limit

A Buy Limit order is a pending order that becomes eligible for execution only when the market price reaches the order’s specified limit price in the intended direction. “Buy” describes the desired side (from the perspective of buying the instrument), and “limit” describes the maximum price you are willing to pay for the fill. To measure a Buy Limit in a way that is independently verifiable, you need to translate the idea into fields you can record from the order ticket and the execution report.

What to measure (core fields)

Measure at least four categories of information:

1. Limit price (L): the exact price value you set on the order ticket.
2. Order timestamps: at minimum, the time of placement (T_place) and the time of any terminal outcome (T_exec for fills, or T_cancel for cancellations). If the platform provides more granular times (for example, “last modified”), record them too.
3. Order state: whether the order is open, partially filled, fully filled, rejected, or cancelled. Partial fills change how you measure completion.
4. Execution outcomes: if and when fills occur, record the executed prices (P_exec_i), executed quantities, and whether there was slippage relative to L.

What “measurement” means in practice

With those fields, you can answer measurable questions like:

• “Did the order ever receive a fill?” (yes/no, possibly with multiple partial fills)
• “How far were executed prices from the limit price?” (difference between P_exec_i and L)
• “What time window was the order exposed while it was open?” (between T_place and the terminal outcome time)

Comparisons that can be verified (and what is not comparable)

To explain how Buy Limit “works” through measurement, compare two timelines:

• Your order timeline: open → partially filled → fully filled (or cancelled/rejected).
• Market crossing timeline (conceptual): the moments when the relevant quoted price reaches or passes the limit price.

A simple, testable metric set is:

1. Exposure duration:
   • Assume a single platform clock. Compute Duration_open = T_terminal − T_place.
2. Fill occurrence:
   • Count the number of execution events n_exec and whether the executed total quantity equals the intended order quantity.
3. Limit deviation:
   • For each fill, compute Deviation_i = P_exec_i − L (using sign conventions you define). You can then summarize deviation (for example, maximum positive deviation or average deviation across fills).

Important limitations for “both options per criterion”

Different platforms and venues may define and report values differently, so treat comparisons carefully. For each criterion, there are two measurement approaches:

• Criterion: price references

  • Option A: use the platform’s reported execution prices (P_exec_i).
  • Option B: use the market quote you observe externally (requires aligning quote time and definition).
  • Limitation: these two can differ even without “wrong” data because quote sources and timing can be different.

• Criterion: time stamps

  • Option A: use the platform’s order and execution timestamps.
  • Option B: use your own system time when you record events.
  • Limitation: clock drift or different time zones can break the “crossing vs execution” comparison.

• Criterion: price crossing

  • Option A: measure crossing using the same feed definition as the platform (if available).
  • Option B: measure crossing using a separate chart or feed you watch.
  • Limitation: historical relationships do not guarantee future behavior; feeds may update at different sampling rates.

Evidence and an example you can reproduce

Below is a measurement example that does not assume real-time data, only recorded fields.

Example setup and assumptions

Assume:

• You placed a Buy Limit at time T_place.
• The order was open until T_terminal.
• The platform later reported one or more executions with executed prices P_exec_1…P_exec_n.
• You define deviation as Deviation_i = P_exec_i − L.

Example calculations

1. Exposure duration

• Duration_open = T_terminal − T_place.

2. Execution count and completion

• n_exec = number of reported fill events.
• Completion check: if total executed quantity equals the intended quantity, the order is fully filled; if not, it is partially filled (before terminal outcome).

3. Deviation summary

• Compute each deviation Deviation_i.
• If any deviation is positive under your sign convention, it indicates executed price higher than the set limit price; if negative, lower.

This is “measuring Buy Limit” because you are using recorded order and execution outputs, not forecasting market behavior.