Quality Score the Most Important in Google Adwords

Quality Score
The adwords system considers the following factors mainly while calculating the quality score.
1) How relevant your keyword to your ad text.
2) How relevant your keyword to user search query.
A quality Score is calculated every time your keywords matches a search query.
Factors Influenced by the quality score:
 Actual Cost per Click
 First Page Bids
 Eligibility of the ad to appear on the SERP
 Ad Rank
Determinants of the Quality Score:
 The historical CTR of the Keyword and Matched Ad
 CTR of the all ads and keywords in the account
 Historical CTR of the Display Url in the adgroup
 The quality of your landing page
 The relevance of the keyword to the ads in the ad group
 The relevance of the keyword and matched ad to the search query
 Account Performance in the geographical region where the ad will be shown
 Other relevance factors

Exceptional Cases:
Ad Position: For calculating the keyword targeted ad position landing page quality is not a factor.
Google Network: Quality score considers CTR on Google + CTR on Search Network
Google: Quality score considers CTR on Google Only
First Page Bid: For calculating the first page bid Quality Score does not consider the matched ad or search query.
 
Quality score for content network:
Determinants:
 The ads past performance on the site and similar sites (i.e. the CTR)
 The relevance of the ads and keywords in the ad group to the site
 The quality of your landing page
Case1: If the bidding is CPM
Quality score is based on landing page quality.
Case2: If the bidding is CPC
Quality Score based on CTR and Landing Page quality
Actual Cost Per Click.
The actual cost per click can be calculated by the following formula
Actual CPC = (Ad rank of your below advertiser/your quality score) +0.01(approximation to the nearest cent)
Ad Rank = (Max CPC*Quality Score)
(Note: the higher the ad rank the higher the position)
Adwords Discounter:
The Adwords Discounter automatically adjusts actual CPC so you pay only one cent more than minimum amount required to keep you ad position.