Criteria and a methodology for evaluating the relative effectiveness of different navigation aids in a coastal region, taking account of the ship-borne equipment, human factors and the needs of the navigator, have been discussed in the Nautical Institute' recently-published book on Pilotage and Shiphandling. The method is based on the product of the probabilities that the equipment will be fitted, that it will be available (in reliability terms), that it will be properly used and that it will provide the required information.