It is impossible to control what can't be measured. If we had no measurements, then no metric would show the effect of a change in a control parameter. The fourth group deals with collecting information about the environment. The measurements, results, and reports are all defined at this group.
A list of metrics might include the number of faults both reported and fixed, lines of code, closeness to plan, resource utilization, and performance. Lines of code is a problematic metric that does not work well in many situations, including composition. Many times the current progress is reported to be on plan, but when the plan indicates a deliverable, it is late.