Explain how implementation shortfall measures the cost of trading

Implementation shortfall compares a paper portfolio, traded instantly at the decision price with no costs, against the actual portfolio. It splits the difference into execution cost for shares filled, opportunity cost for shares not filled, and explicit fees. Example: a manager decides to buy 10,000 shares at 50.00, fills 8,000 at an average 50.30, cancels the rest when the price reaches 51.00, and pays 400 in commissions. Shortfall = 2,400 + 2,000 + 400 = 4,800 (96 bps of the 500,000 paper investment); execution cost is 8,000 × 0.30 = 2,400 and opportunity cost is 2,000 × 1.00 = 2,000. Its strength is capturing all costs, including unfilled orders. A trap is using the arrival price instead of the decision price.