Google Quality Score

According to Google, your Quality Score is based on the following elements  

Expected Keyword Click-Through Rate (CTR) Metric calculated based on past CTR and historical performance of the keyword in your ad 

Display URL s Historical CTR How often your display URL receives clicks 

Landing Page Quality How relevant your landing page is and how easy it is to view 

Keyword/Search Relevance How relevant your keyword is to what a 

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