Source: http://www.mercer.com/referencecontent.htm?idContent=1307990#Top50_qol

From Mercer:

The Quality of Living index is based on several criteria used to judge whether an expatriate is entitled to a hardship allowance. A city with a high Quality of Living index is a safe and stable one, but it may be lacking the dynamic je ne sais quoi that makes people want to live in world-renowned cities such as Paris, Tokyo, London or New York. Sometimes you need a little spice to make a city exciting. But that "spice" may also give a city a lower ranking.

What makes one person's quality of life better or worse cannot be quantified in an objective index. Therefore, Mercer's Quality of Living report reflects only the tangible aspects of living in a city on expatriate assignments, and leaves the question of the quality of one's life to those living it!