Jens Ludwig emphasizes that benefit-cost analysis must be based on program evaluation, which identifies the causal effects of the policy or program that is the focus of the analysis. In attaching costs to the impacts of a particular policy or program, two important costs that are often overlooked are "opportunity" costs and the the "intangible" costs of crime. " Opportunity" costs are the costs of the program conducted compared to the costs of an existing or alternative program or policy. The "intangible" costs of crime are fear and other adverse emotional reactions to crime that undermine the quality of life and socioeconomic opportunities of the victim.