XP discusses two other roles. They might not be present in a formal capacity on every team.
The tracker keeps track of the schedule. XP tracks a few metrics. The most important is team velocity, which is the ratio of ideal time estimated for tasks to the actual time spent implementing them. Other important data may include any changes in velocity, the amount of overtime worked, and the ratio of passing tests to failing tests.
All of these numbers measure progress and the rate of progress. They help determine if the project is on schedule for the iteration. They can signal behavioral changes that may affect the schedule. Looking at the numbers alone rarely gives the whole picture; anomalies should be brought before the whole team for analysis during the stand-up meeting (see Chapter 9 in Part III.)
To measure velocity within the iteration, every day or two, the tracker asks each developer how many tasks she has completed. This is best done in person, as informally and comfortably as possible. Honesty is vital on the part of developers, and the tracker should be nonjudgmental. This may be a manager or a trusted developer. Regularly tracking progress helps the team adjust to its ebb and flow of work.
Some XP projects have a coach who guides and mentors the team. This can be helpful when adopting XP. His position is one of respect—he leads by example.
XP can be difficult to apply consistently. Though many of its practices are common sense, ...