Once a component is completed to the satisfaction of the team, it is placed under change-order control. When a component is placed under the change-order control process, changes to the component are now frozen. If a change is absolutely necessary, and the teams are willing to delay the project to enforce the consequences of the change, then the change is fulfilled. Changes should be few, well justified, and documented.
Obviously, early in the process, analysis tasks are naturally a high priority. Later in the process, testing and quality become a higher priority. This is where the change-order control process becomes important. At the beginning of the process all four categories of analysis, design, implementation, and testing are available for prioritizing and scheduling. At the P1-P2 transition point, see Figure 3.4 on page , in the process, the analysis phase is subjected to change-order control process. Having the analysis phase frozen focuses attention on the remaining three categories of tasks. In a similar fashion, at the P2-P3 transition point, see Figure 3.4 on page , the design phase is frozen and at the P3-P4 transition point the implementation phase is frozen. At the final stage only changes that affect quality are allowed. This leads to a definition of temporal stages in the methodology, specifying priorities.
Don't confuse phases with stages. A phase is a grouping of similar activities. A stage is a temporal grouping of tasks within phases at particular times. Stages follow one another.