As the name implies, the B–C ratio method involves the calculation of a ratio of
benefits to costs. Whether evaluating a project in the private sector or in the public
sector, the time value of money must be considered to account for the timing of cash
flows (or benefits) occurring after the inception of the project. Thus, the B–C ratio
is actually a ratio of discounted benefits to discounted costs.
Any method for formally evaluating projects in the public sector must consider
the worthiness of allocating resources to achieve social goals. For over 70 years, the
B–C ratio method has been the accepted procedure for making go/no-go decisions
on independent projects and for comparing mutually exclusive projects in the
public sector, even though the other methods discussed in Chapter 5 (PW, AW,
IRR, etc.) will lead to identical recommendations, assuming all these procedures are
properly applied.
Accordingly, the purpose of this section is to describe and illustrate the
mechanics of the B–C ratio method for evaluating projects. Two different B–C ratios
will be presented because they are used in practice by various government agencies
and municipalities. Both ratios lead to the identical choice of which project is best
when comparing mutually exclusive alternatives.
The B–C ratio is defined as the ratio of the equivalent worth of benefits to the
equivalent worth of costs. The equivalent-worth measure applied can be present
worth, annual worth, or future worth, but customarily, either PW or AW is used.An
interest rate for public projects, as discussed in the previous section, is used in the
equivalent-worth calculations. The B–C ratio is also known as the savings-investment
ratio (SIR) by some governmental agencies.
Several different formulations of the B–C ratio have been developed. Two of
the more commonly used formulations are presented in this section, illustrating
the use of both present worth and annual worth.