What contributes to a successful monitoring system?

A Monitoring system is an essential component of a particular Management System, e.g. Operation Management System, Environmental Management System, Integrated Environmental Management System, etc. 

The monitoring system within any management system is considered to be successful, when all the key performance indicators of the management system are measurable in line with the concerned management system requirements. The major attributing factors for the success of any monitoring system, hence, would be - expertise of the monitoring team, effectiveness of measurement devices, and fulfillment of monitoring requirements.