The techniques of compiler optimisation can be applied to aid a programmer in writing a program which cannot be improved by these techniques. A finite, representative set of test data can be useful in this process. This paper presents the theoretical basis for the (nonconstructive) existence of test sets which serve as maximally effective standins for an unlimited number of input possibilities. It is argued that although the time required by a compiler to fully exercise a program on a set of data may be large, the corresponding improvement in the reliability of the program may also be large if the set meets the given theoretical requirements.