The Functional Levels of Correctness

(derived and edited from a PL/1 reference)

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LEVEL I:
The program contains no syntax errors that can be detected by the compiler.

LEVEL II:
The program can be executed.

LEVEL III:
There exists a set of input for which the program will yield the correct answer.

LEVEL IV:
There exists a set of typical valid input for which the program will yield the correct answer.

LEVEL V:
There exists a set of deliberately difficult input for which the program will yield the correct answer.

LEVEL VI:
For all sets of input, valid with respect to the original specification of the program, the program returns the correct answer.

LEVEL VII:
For all possible sets of valid input, and for all likely sets of erroneous input the program returns a correct or reasonable answer.

LEVEL VIII:
For all possible sets of input (valid and erroneous) the program gives a correct or reasonable answer.

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