The progressive utilization of information necessitates the evaluation of the information‐content of scientific documents over conventional bibliotechnical indexing‐operations. The technique of optimal economical evaluation of information rests in the alternative between the keyword and subject‐index as the symbolic operator. From the analysis of 350 scientific documents the keyword‐concept was, by empirical decision, derived to be the superior operator. Conversely, the concept of subject‐indexing leads to ambiguities of subcategories, since certain branch‐nodes of the categorical trees (code trees) may belong to several simultaneous subject‐categories. This ambiguity can be resolved only by means of coordination‐tables correlating each keyword with the pertinent simultaneous subject‐categories. The application of synonym‐registers affects the concentration of information‐sources with typical completeness for each keyword‐entry in the system. The phrasing of syntactically associated keywords produces a significant precision of the information‐content of the documentary text. The efficiency of a documentary system is predominantly accomplished by information‐filtering in terms of relevant keyword‐phrases. This system‐efficiency is displayed as a function of utility‐rate versus precision‐rate, which is considered superior to Salton's efficiency‐rate of “precision versus recall.” The phrasing of key‐word‐phrases is utilized as the structure for the design of a truly topic‐related abstract to any document. Empirical findings from this study of economical optimization of documentary systems reveal the actual efficiency of the concept of information‐evaluation.
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