This chapter describes why evidence-based crime prevention and policing advocates are making a logical fallacy. It shows why we cannot take the results from meta-analysis as indicators of what policymakers should do. The chapter also describes how we can use evaluative research in a more modest, but nevertheless important way. It analyzes the notions of deduction, induction and falsification. The chapter explores how falsification applies to internal validity but not to external validity. It also shows how commonly described evidence-based practices ignore this distinction between internal and external validity and presume to make general statements based solely on internal validity criterion. The chapter discusses how we can generalize by starting with a strong theory. Theories are general statements or conjectures about how some bit of the world works. The chapter also discusses how using external validity concepts may provide policymakers with better information upon which to make decisions.