After choosing the classification of project inputs and impacts, the next step is to establish the causal relationship between the two. The question is, how do we know that the project made a difference in performance? In some cases, the public investment analyst works with production functions supplied by technical experts. For example, for an agricultural project, the input-output data may come from agronomists and engineers, for a health project, from physicians, and for an educational project, from educational psychologists. But in other cases these data are not available, and the budget analyst must participate in securing the required data. The analyst must at least know how to interpret production function data; this evaluation is the main purpose of this chapter.