The statement of work did not sufficiently detail the requirements for the online help system. The initial project planning needs to include a scope statement along with product requirements. Creating an online help system is far too generic and means different things to different people.
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As we designed and built the prototype and demonstrated functionality to the stakeholders, they began asking "what about this" or "can we add that" and every time a new version was presented, there were more requests made. There was no process or documentation created for change requests -- it was primarily verbal suggestions made during demonstrations or a quick email request.
In the future, I would ensure proper planning occurs before any work on the project begins. The planning process would also define how change requests will be processed, including the need for approval before any change is introduced.