This paper describes the development of a Decision Support System, from its beginning as part of a research project through to the implementation of it as a commercial product used by six state agencies and public sector consulting groups. The system was designed by individuals with a long-standing involvement with Dss. As such, it provided an excellent opportunity to test the conventional wisdom on principles and techniques for Dss design.<br><br>We had clear expectations as to what would be easy and what would be hard to implement. We wanted to see if the Dss field is at a stage where one can give builders reliable rules of thumb not a cookbook, but the sort of pragmatic advice that would be welcomed by a capable systems analyst, consultant or programmer setting out for the first time to deliver an interactive computer system to support decision makers in a complex task.<br><br>Isspa (Interactive Support System for Policy Analysts) is a Dss, written in Apl, that supports administrators, analysts and researchers concerned with public policy issues at the state and local level. The initial application which this paper discusses is in the area of school finance: the funding of public education in individual states. However, Isspa is of general relevance to planning and policy making in both the public and private sectors.<br><br>The development strategy was based on principles of adaptive design, derived from the recommendations of several researchers and practitioners (see Section 5).