[dschoon]

This critique reminds me very strongly of Evan Miller's "How Not To Sort By Average Rating": http://www.evanmiller.org/how-not-to-sort-by-average-rating....

In both cases, the key observation is to realize that the meaning of the numbers is more important than the numbers (or how they're calculated, per se). This point is kind of abstract--especially compared to the salient examples in both pieces--but if you're looking to understand why this error is so common, look first to the fact that it's not an error of stupidity or insufficient maths.

You can sometimes avoid a big practical problem (e.g., an avenue to attack your system via diluting meaning ("spam")) by abstractly considering the meaning behind the structures available to you.