A method of reaching economic decisions by comparing the costs of doing something with its benefits. It sounds simple and common-sensical, but, in practice, it can easily become complicated and is much abused. With careful selection of the assumptions used in cost-benefit analysis it can be made to support, or oppose, almost anything. This is particularly so when the decision being con templated involves some cost or benefit for which there is no market price or which, because of an externality, is not fully reflected in the market price. Typical examples would be a project to build a hydroelectric dam in an area of outstanding natural beauty or a law to require factories to limit emissions of gases that may cause ill-health. (See shadow price. )