This used to be a technical question I'd use in the first face-to-face interview but I was stunned to find that some candidates either

a) Did not know what QS was or

b) Thought I was talking about PageRank!

While I do not expect Google's textbook answer, I am hoping the candidate understands the basic concept behind QS (clickthrough rate + other factors) and that QS is part of the Ad Rank formula where:

Ad Rank = CPC bid X Quality Score.

I now use this question to open the technical section of a phone interview. A primary function in their role as PPC analyst will be to educate non-search people. Their ability to explain Quality Score and Ad Rank to me in plain English is something I pay very close attention to.