The B/C is the ratio of the present worth of benefits and costs (discounted for the same discount/interest rate). Alternatives which have a B/C ratio larger than 1 are considered as profitable.
The IRR, is the discount rate at which discounted benefits equal discounted costs. Decision on profitability is based on a comparison of the IRR with a minimum interest rate (a political choice).
Costs and benefits of alternatives are compared with the reference situation (in most cases the zero-situation). To value these in money, economic prediction models can be used. It is, however, difficult to estimate the benefits satisfactorily. Some benefits can not be expressed in money and will be described qualitatively. The values of such benefits are indicated in the effect overview as PM. In such cases, the effect overview does not give a full indication of the effects, so that an unequivocal ranking of alternatives can not be obtained.
A special monetary method is a cost-effectiveness analysis, which can be used of identifying the alternative that:
A cost-effectiveness analysis can not be used for indicating the rentability of alternatives; only for ranking.
In translating non-monetary values of for instance nature reserves, sometimes use is made of replacement values. Here, questions need to be answered like: "How much is the willingness to pay (from society) for maintaining the nature reserve or creating a new one".