Heard of Goodhart’s Law? It says, when a measure becomes a target, it ceases to be a good measure. It is an idea that was first mooted by economist Charles Goodhart, in 1975, and later used widely to criticise Margaret Thatcher for trying to conduct monetary policy based on targets set to regulate broad and narrow money. Goodhart noted that any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes.