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The basic idea of Halstead's (1977) theory was to distinguish the operators and the operands inside a program written in any programming language. Johnston and Lister (1981) observed that Halstead's relation underestimated the program length, particularly with programs written in such highly structured languages as PASCAL. Johnston and Lister noticed a lack of operators resulting from the structure of this language, and they proposed a modification to Halstead's formula. In an experiment, an examination was made of approximately 550 PASCAL programs in the data processing center of the University of Udine. The results represent the widest test of Halstead's theory with regard to PASCAL programs. The experiment confirmed the results of Johnston and Lister but suggested a correction to their formula. This correction is particularly necessary for large programs.