A passive order’s lower spread cost can come with incomplete execution

Separate order instructions from outcomes

The SEC’s order-type guide explains that a market order prioritizes execution at the available market price, while a limit order specifies a price condition and may remain unexecuted. Those distinctions do not establish that one order type is always cheaper for a particular investment objective.

A passive order scenario needs assumptions about where the order rests, how much executes, the time allowed and what happens to the remainder. A price-only spreadsheet can make waiting look costless because it never records the investment exposure that was not acquired or the risk that was not removed.

A complete hypothetical comparison

Assume a buy instruction for 10,000 shares, a $100 decision reference and a one-hour completion horizon. Scenario A buys all shares at $100.05 with $100 fees. Its benchmark-relative cost is $500 + $100 = $600, or 6 bps of the intended $1 million.

Scenario B fills 4,000 shares at $99.98 with $40 fees, while 6,000 remain unfilled. At the evaluation time the price is $100.20. Filled-price improvement is $80, but missed exposure is $1,200. Total shortfall under this guide’s convention is −$80 + $40 + $1,200 = $1,160, or 11.6 bps. These invented outcomes illustrate the accounting; they are not forecasts of either order type.

The opposite price path can reverse the comparison

If the evaluation price in Scenario B were $99.80, missed exposure would be −$1,200 and total shortfall would be −$1,240. A passive approach can look much better on that path. The comparison therefore depends on future price movement and completion, not just an assumed rebate or saved half-spread.

A fair empirical comparison needs many eligible orders and a predeclared evaluation method. Selecting only fills or only days on which the price moved favorably creates a biased sample. Retain cancelled, expired and unfilled instructions, and distinguish deliberate cancellations due to new information from failure to obtain the originally requested quantity.

Resting liquidity is exposed to selection

An order may fill precisely when another participant is willing to trade at its limit. Subsequent price movement can be relevant to the quality of that fill. A simplified adverse-selection study could record the signed price change after each fill at a fixed horizon, with the horizon chosen before reviewing the results.

That study still would not identify all causes of the movement. News, market drift and the original signal may affect the same prices. Keep observed post-fill movement separate from a causal claim that the order type produced it. The workbench’s fixed half-spread allowance contains none of these queue or selection dynamics.

Operational evidence matters

FINRA’s best-execution rule includes considerations beyond a single quoted price, including execution likelihood and order characteristics. For this publication, that supports inspecting completion and timing alongside price. It does not confer any broker duty or regulatory approval on the educational calculator.

A practical review packet can include limit price, order receipt time, route, cancellations, fills, remaining quantity and any replacement instruction. If those records are missing, say that fill-only price analysis is incomplete. Never convert a small observed spread cost on selected fills into an unrestricted strategy-capacity claim.

Keep in the decision record

• A limit price does not guarantee completion.
• Compare all intended quantity over a declared horizon.
• Preserve unfilled and cancelled instructions in empirical samples.

Source notes

1. SEC Investor.gov — Types of Orders
2. FINRA — Rule 5310, Best Execution and Interpositioning

Sources inform the definitions and limitations. Scenario inputs and arithmetic are publication illustrations, not extracted market observations. Access checks are dated in the source inventory.

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