One of the most serious theoretical and practical limitations of the Entity Relationship (ER) Model is its inability to cope with complexity. Once data models exceed a certain threshold size, they become difficult to understand, document and maintain. A number of approaches have been proposed in the literature to address this problem, but so far there has been little or no empirical validation of these methods in practice. This paper describes an action research programme in which a method for representing large data models was tested in eight different organizations over a seven year period. Action research was chosen as it provides a way of validating the method in a real world context using real practitioners. The method evolved significantly as a result of experience in practice, and is now a stable and mature method, as evidenced by the lack of change to the method in the last three action research studies.