When I was a young lad my grandfather would tell me stories about working in the coal mine. The stories were often quite funny and full of more characters than a Disney movie. A lot of the stories touched on the never ending struggle between management and workers. During this time there were a lot of experiments focused on performance measurement. How can we get the miners to dig for more coal?
One method that was employed was to pay miners by how far they dug into the coal seam. The more they dug, the more money. Progress was tracked with an official 'measuring stick'. One day, either by accident or deliberate (it remains a topic of great debate), a part of the top of the measuring stick was cut off. As a result the miners were getting credit for digging more than they actually did. This went on for some time. Eventually a manager was in the mine shaft and needed to measure something. He requested the official measuring stick and quickly realized what happened.
The moral of this story?
No matter how clever you are, how hard you try, and how tight you control things, someone will find a way to work around it in very short order. It is why performance measurement should not be focused on control, but on improvement. And those that are being measured need a means to contribute to the improvement and an incentive to do so.
Otherwise - no metric can beat us!