The prediction in Health Impact Assessment must be evidence based. Predictions are widely used to inform decision making in other fields. Economists predict, with varying degrees of accuracy, the effect of fiscal measures on trade and economic well-being, and the effect of price and income on sales of products such as cigarettes and alcoholic beverages. Climatologists predict the effect of energy policy on future climatic conditions. Transport planners predict the effect of road and railway developments on traffic, mobility and quality of life. Urban planners predict the effect of the built environment on quality of life for urban residents. Risk assessment engineers predict the likelihood of catastrophic events in safety-critical structures such as aeroplanes, chemical plants and atomic power stations.