All members of the group can change any code at any time. XP Principle Interdependencies Group ownership depends on frequent testing, continuous integration, coding standards, and pair programming. Refactoring depends on coding standards, group ownership, continuous integration, pair programming, and simple design. Simple design depends on communication, refactoring, and pair programming. Testing depends on simple design, and pair programming.
Pair programming depends on correct working hours, coding standards, simple design, and testing.
There are other dependencies, but these illustrate the point. Small changes are made since big changes are more risky and may fail. Teach strategies to help staff learn how much testing should be done. Keep initial project investment small - Start with a small team and build up in size allowing members to learn from each other. Do experiments to eliminate the risk of technology or incorrect decisions.