The need to envision and architecture data base systems in a hierarchical level by level framework is stressed. The inverted data base (file) organization is then analyzed, considering implementation oriented aspects. The inverted directory is viewed realistically as another large data base which itself is subjected to inversion. Formulations are derived to estimate average access time (read only) and storage requirements, formalizing the interaction of data base content characteristics, logical complexity of queries, and machine timing and blocking specifications identified as having a first-order effect on performance. The formulations presented are necessary to be used in conjunction with any index selection criteria to determine the optimum set of index keys. © 1975, ACM. All rights reserved.