Inherent in the whole of the above analysis is Wilkes' assumption
that money, i.e. nominal, rates of return are more constant than real rates
and are, therefore, more readily estimated. If this assumption is invalid,
and the investment opportunity cost could be estimated more accurately
in real terms, then double discounting would be the preferable approach
to evaluation. The basic concept of the use of an investment opportunity
cost rate is not, however, in question.