Explain how the cost of a trade is measured and work out that cost

Implementation shortfall compares a paper portfolio traded instantly at the decision price with the real one. It splits into delay cost (arrival price − decision price, on shares traded), execution cost (execution price − arrival price, on shares traded), opportunity cost (final or cancellation price − decision price, on shares never traded) and explicit fees. Example: decide to buy 1,000 shares at 20.00; arrival price 20.05; 800 shares filled at 20.10; the rest cancelled with the price at 20.30; commission 40. Delay 800 × 0.05 = 40, execution 800 × 0.05 = 40, opportunity 200 × 0.30 = 60, fees 40: total 180, or 90 bp of the 20,000 paper investment. The trap is forgetting the unfilled shares.