medium · Market Microstructure
A sell-side quant explains that an execution algorithm requires 20 to 30 trading periods to reliably estimate the permanent price impact component of a large order.
Which explanation best captures why so many periods are needed?
- Permanent price impact is always exactly half the quoted spread, so 20 to 30 periods are needed only to compute the average spread reliably.
- Execution algorithms require 20 to 30 periods because Reg NMS mandates a minimum evaluation window for any reported implementation shortfall figure.
- Permanent price impact is estimated from the difference between post-trade and pre-trade midpoints; the short-run price move is dominated by temporary impact and mean-reversion noise, so many independent observations are needed to isolate the smaller permanent signal with statistical power.
- The permanent component is estimated from the VWAP of the day, which requires the full trading day (typically 20 to 30 five-minute intervals) to compute.
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