6.14 When there is no market price for costs and benefits to society they need to be estimated and are known as shadow prices. This is particularly important for environmental, social and health effects. Some have generic values generated, for example, through surveys of a sample of the population. These are included, with information on how to use them, in Annex 2 and the Green Book webpages. To ensure appropriate use it is important to understand the difference between the characteristics of the sample population and an intervention's intended target population. The advice of professional economists is required when dealing with non-market valuation.