Arrival-Price Algorithms

Most execution benchmarks are measured against something that happens during the trade — the day's volume-weighted average price, for instance. Arrival price is different: it freezes the price at the single instant the order arrived at the desk, and the algorithm's entire job is to trade close to that frozen number, no matter how the market moves afterward. It's the natural benchmark for Implementation Shortfall, because it isolates exactly the cost of acting on a decision from the decision itself.

A worked walkthrough

A portfolio manager decides to buy 100,000 shares at 9:30:00, when the stock is trading at $50.00 — this is the arrival price, locked in immediately. An arrival-price algorithm now has to choose how fast to trade.

Fast (high urgency): it executes 60,000 shares in the first 20 minutes, pushing the average fill to $50.04 as it walks the book repeatedly, then finishes the remaining 40,000 more gently by 10:30 at an average of $50.03. Blended average fill: $50.035. Slippage against arrival: $0.035, i.e. $3,500 on the full order — mostly market impact from trading fast.

Slow (low urgency): it spreads all 100,000 shares evenly over four hours. But the stock happens to drift up steadily to $50.10 by early afternoon on unrelated buying interest, and the algorithm's average fill ends up $50.06. Slippage against arrival: $0.06, i.e. $6,000 — mostly market drift ("timing risk"), not impact, because trading slowly left the order exposed to the market moving away for longer.

The two scenarios show the actual tradeoff: trading fast concentrates cost into impact you cause yourself; trading slow trades that for exposure to a market that might move against you anyway. Neither is free — the algorithm's urgency setting is a bet on which risk is larger for this order, in this stock, right now.

Where it's used. Arrival-price algorithms are the default for orders carrying real alpha, where the signal that triggered the trade is expected to decay — every minute spent trading slowly risks giving back edge to the drift the signal predicted, so protecting the arrival price (not chasing VWAP) is the priority. See Urgency And The Risk-Aversion Parameter for how the fast/slow tradeoff above is actually optimised rather than guessed.