Goodhart's Law: That which is formally measured becomes useless as a measure.

[richardhutnik]

shuraiya said:

Human nature; it can't be helped. It's pretty much why blind/double blind experiments are all but necessary in many scientific studies.

And also what happens is that the political realm will end up changing the measure in order to make things look better.  Like take a look at the government measures in USA for inflation and unemployment:

http://www.shadowstats.com/