Google Publishes Quality Score Explanation

Google just published updated information on ‘Quality Score’ and how is it calculated. Quality Score is a major factor in determining what advertisers pay for ad phrases.

I believe this is the clearest information so far on this import, evolving algorithm.
What is ‘Quality Score’ and how is it calculated?

Past Articles:

• How Google AdWords Bidding Works – Quality Score
• Landing Pages to Improve Quality Score
• PPC Campaigns and Quality Score
• Chasing the Long Tail