This is technically a "reshare" from a while back. Google released this handy white paper on their meaning and usage of quality score. I get so many clients that are stuck on this number as a stand-alone metric. That is not the intent of quality score.
My favorite point from the white paper is that QS is NOT a Key Performance Indicator (KPI). It is simply a diagnostic tool. It's great to look at, but always remember that your end goal is conversions at a profitable CPA.
Another key quote (that many debate) is on page 10! There is no such thing as an ad group, campaign, or account level quality score!
Enjoy and Share.