This paper is a reflective paper on practical. experiences. It presents a project model that is in current use and has been successfully used in several large projects.

The Mentor project model supports experimental development of contract software. The application area is interactive information systems, i.e. systems closely integrated with user work practice.

The Mentor project model is a spiral model with iteration of the activities: (re- )design, estimation and negotiation, development and evaluation of prototypes. End-users are actively involved in design and evaluation. Repeated estimation and negotiation activities based on a calculation model for estimating system extent ensure that growth in extent is made visible and is subject to explicit decisions balancing use quality of the system with cost and schedule. The project model defines how to share the cost of experimentation between customer and supplier.

The model contributes to the ongoing discussions of system development methodologies by focusing on contract software and by giving elaborate suggestions for how to prototype large information systems with many users.