Google on Tuesday issued a clarification and extensive explanations regarding its Quality Score - used to calculate an advertiser's minimum bid.

The previously vague guidelines now instruct advertisers on how they can improve their quality scores, writes MediaPost. Google incorporated quality scores last year based on prior ad performance and this summer refined its methodology by taking into account landing-page quality.

A new Ad Quality and Performance section in the Help Center offers a Quality and Performance Overview; Quality and Performance Factors; Troubleshooting; and Improving Ad Performance.

"The guidelines…aren't hard-and-fast rules, nor are they exhaustive. However, they do reflect the site quality principles we'll incorporate into factors such as ad approval status and Quality Score. So, following these guidelines, when appropriate, will improve the performance of your AdWords advertising," Google writes

Separately, Google banned AdSense publishers from displaying images near their text ad units - because, it said, consumers were clicking on the ads by mistake, thinking they were associated with the images.