This thesis describes a two part research program in which the applicability of subjective ratings to the selection of independent variables was evaluated. The first portion of the research reviewed a case study involving the application of complex system investigation to the development of a telephone inquiry system. A telephone inquiry system is one in which users seek information in a data base by calling the system, listening to information presented by a synthetic voice, and directing movement through the database with commands on the telephone keypad keys. The complex system investigation method used included identifying the independent variables by brainstorming, then reducing the list by subjecting the variables to literature review, feasibility analysis, relevance analysis, and subjective ratings of the factors based on a prototype system. Variables which were not likely to have an immediate impact on human performance in the system were set to a constant value. The use of subjective ratings to select independent variables stems from the need to reduce large numbers of independent variables to a list which can be used as candidates for a screening study. The result of the case study was a list of 19 candidate factors suggested for implementation in a screening study. The second portion of the research describes an experiment in which 5 independent variables ( number of steps in a search, adapting speech rate, transaction summary, native/non-native, and sex of the voice) were chosen to represent the 19 candidate factors in an experiment testing the validity of the ·subjective ratings technique. The results indicated that the subjective ratings of the prototype system were effective in predicting performance and subjective ratings. The impact of these results on the methodology and telephone inquiry systems is also discussed. / M.S.