Gall's Law

Complex systems evolve from working, simple systems

Working Systems, Budgets, and Scheduling

In the 1975 book “Systematics” John Gall observed

A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system.

I think this is eactly correct. Complex systems must be built on top of robust and well designed infrastructure. Successful complex systems are not the result of a Big Bang style massive-build-then-deploy strategy. Instead, succesful complex systems are the result of a sequence of small projects, each building on the success of the earlier projects. Estimating the cost and schedule of a complex system is very much hit or miss. Estimating the cost and schedule of a small project is a much more well-defined process. Therefore, while one can make a “best guess” at the budget and schedule for a complex project, this shouldn’t be considered a hard target unless the feature set for the complex project is flexible. As each small project becomes clearly defined, refine the overall budget and schedulei (or feature set) for the complex project. Be very aware that as the smaller systems feature sets become well-defined and their budgets and schedules are made, the complex system endpoint budget and schedule (or feature set) will need to be revised.

I would say the real value in the initial budget, schedule, and feature set is setting the context for understanding the limitations and goals of the ‘client.’ Decisions and tradeoffs that will inevitably occur as development proceeds are made in the context of the initial requirements, budget, and schedule.