Measurements are the primary method of accounting for the results of actions being taken within an organization. As such, they can be considered to be an output of the system.
Measurements are the primary source of feedback to managers and workers alike.
However, measurements also act as an input . They have a huge influence on the actions that employees chooses to take in the first place. And, as the Goldratt quote above says, measurements that are not clearly understood will cause the system to experience chaos. This is why TOC practitioners often begin by examining the measurements being used by an organization.