Tanmay Arora, a Google representative focused on AdWords performance, posted on the Google AdWords Help forums a list of "ingredients" that make up the Google AdWords Quality Score.
Here they are:
Keyword Relevance Not About Keyword Density
The relevance of a keyword is not entirely determined by its presence on the landing page or the number of times it's been mentioned on the landing page. It’s not about how appropriate we find the keyword to the product/landing page but how appropriate the users find it. In other words, the number of users clicking on your ad when they search for that keyword.
New Keyword Get Historical Quality Score Until They Earn Their Own Stripes
When we add fresh keywords, initially, they’re awarded a historical Quality Score based on their previous performance on Google.com. And only once the keyword starts accruing statistics, the system then evaluates its Quality Score based on its recent performance. This doesn’t happen dynamically but is a gradual process.
CTR Is Based Off Exact Match Keyword
We take into account the exact match CTR of the keyword, as it’s a better indicator of the effectiveness of the keyword. (The exact match CTR refers to the number of times the keyword has triggered an ad when the search term exactly matched the keyword.) For example, if our keyword ‘red shoes' is in broad match, it triggers our ad even for search terms like 'red shoe’, ‘formal shoes’, ‘horse shoe,’ etc. However, the exact match statistics point out exactly when the keyword ‘red shoes’ triggered our ad and was clicked on by the user when he searched for the exact search term ‘red shoes’.
The More Impressions, The More CTR Matters For Quality Score
Besides the performance, the number of impressions for a given keyword also plays an important role in defining the Quality Score. This means that as more and more performance history keeps building up (in terms of impressions) at your account level, URL level, and keyword level, CTR starts playing a much bigger role in defining the Quality Score.
Those are the nuggets of information she shared at the Google AdWords Help.
Here is a video on Quality Score from Google as well:
Hat tip to Kim Clinkunbroomer for sharing this with me.
Forum discussion at Google AdWords Help.