For instance, production costs are dependant on numbers of units produced. Often the choice is not about whether to carry out a certain intervention or not but about varying the volume of services currently provided or shifting resources between services. The comparison then must be based on inputs necessary at that level of variation and the change in consequences that result from that variation of inputs. This type of logic is called marginal and is at the basis of all the costing procedures which economists use. All comparisons made within the framework of an economic evaluation are based on marginal logic.
Evaluations are models which attempt to capture and summarise reality.
Where data is absent or uncertain, the gap may have to be filled by assumptions. In order to deal with uncertainty and carry on with our decision-making process, economic evaluations use a range of techniques, called sensitivity analysis, which repeats the comparison between inputs and consequences varying the assumptions underlying the estimates. In other words it tests the robustness of the conclusions by varying the items around which there is uncertainty.