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I would also add the importance of no-followed links. Based of what I've seen around a percentage between 2% and 10% of no followed links makes your website "natural". Moreover, I'd mention the importance of the proportion of internal and external links. Again, based on my experience (not official and tested data) a natural proportion would be around: 7%-25% external and rest internal. We could make an important distinction: are we talking about "root domain", "sub-domain" or "page level"? The proportion of the link ingredients is different for each of them...