The quality score concept is difficult to understand in paid search before we start on how to improve Quality score we should understand the quality score concept.
What is Quality Score?
Google definition for the quality score:
“Quality Score is an estimate of how relevant your ads, keywords, and landing page are to a person seeing your ad. Having a high Quality Score means that our systems think your ad, keyword, and landing page are all relevant and useful to someone looking at your ad. Having a low Quality Score, on the other hand, means that your ads, keywords, and landing page probably aren’t as relevant and useful to someone looking at your ad.”
The critical factors that affect Quality score

• Past CTR for your keywords
• Past CTR for your display URL
• The overall CTR history for your account
• Landing page (Destination URL)
• Keyword relevance

The importance of Quality score

• Reduce CPC (cost per click)
• Improve ad position

How to optimize Quality Score

• Reduce the number of keywords per ad group

Try to make a short list of keywords for every ad group to increase quality score, ad relevance and CTR.

• Make a negative keyword list

The unwanted keywords damage the CTR so you should use Adwords keyword tool to know the unwanted keywords and put them in the negative keywords list.

• Improve landing page

Relevant and original content, transparency and ease of navigation are important factors to enhance your landing page to increase your quality score.