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Does a good SEO Score (over 90% for example) guaranties a better ranking on Google (or yahoo or others)?
There is no such thing as an SEO score. Whomever gave your site an SEO score made it up using criteria they deemed important. This in no way, shape, or form has any effect on nor is any indication of how well you will rank.
There is no such score...but there are tools out there that will assign an SEO score to your website.
And yes in my experience those with a higher score tend to rank better and get more traffic.
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