Maintenance of this web site is currently subsidised by unrelated business activities. Go to Bytesmiths Press for details. Again, developers take a conceptual specification and develop the product, constrained by legacy systems and market opportunity windows. In addition to assuring customer satisfaction, maintainers produce reusable test cases, and provide early and continuous feedback to the development activity. Although not expressible in IDEF0 syntax, the feedback loops are negative feedback , meaning they tend to minimize the impact of the inputs, resources, and constraints.
Negative feedback is a stabilizing influence.
Waterfall feedback tends to be positive , or amplifying feedback, due to the long periods involved. For example, developers are ramped up on another project by the time defects come in from testing, so they divert resource to bug fixing, which subsequently reduces the quality of their present work, which increases the number of defects. Let's look at the spiral development activity in more detail.