Implementation Shortfall
Implementation shortfall is the gap between a decision price (or arrival price) and the actual average execution price, including missed-trade opportunity cost.
Definition
Implementation Shortfall refers to trade opportunity cost. Keep that definition fixed when comparing series, managers, or regimes — renaming the same tape does not create a new signal.
Why it matters
Price formation at the venue layer decides whether a signal survives implementation. When trade opportunity cost shifts, related hedges, limits, and narratives usually need an explicit update rather than a quiet assumption.
Case
Suppose a desk is positioned for the opposite of what implementation shortfall is saying. If trade opportunity cost moves against that book, the first question is not “is the story clever?” but whether size, hedges, and stop logic still match the observation.
How to read it
Measure spread, queue, and impact at your size; paper fills are not a desk edge. Prefer a short written null hypothesis for Implementation Shortfall: what would falsify the current reading in the next window?
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