Counting things is generally much easier than defining sharply the boundary between what is to be counted and what is not. Focus often lands on that problem late, when a decision is to be made based on (or at least informed by) the metric and a shift in the definition could change the outcome.
While more attention early is definitely better, no amount of sharpening is ever going to anticipate every variation that will arise. That’s why it is so important to use metrics in the context of the questions they are helping answer and the goals being sought.
http://www.nytimes.com/2015/12/04/opinion/how-many-mass-shootings-are-there-really.html?_r=0