i just found this,
How is the Quality Score calculated?
We want to ensure that your keywords get a fair chance to run and that we do all we can to properly gauge their performance. We use a Quality Score to do this. Each keyword is given a Quality Score based on data specific to your account, including your keyword's clickthrough rate (CTR), relevance of ad text, historical keyword performance, and other relevancy factors.
Quality Score = keyword's CTR + relevance of your ad text + historical keyword performance + other relevancy factors
Your keyword's Quality Score and maximum CPC (at the keyword or Ad Group level) determine your ad's rank on Google search and content sites. (For the top positions above Google search results, however, we use your keyword's actual CPC.) Remember that improving the relevance of your ad text and keywords will increase your keyword's Quality Score and reduce the price you pay when someone clicks on your ad.
so, from this, it is the individual keyword, not the adgroup.