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The truth is no one knows... That's why the algo is such an evolving mystery It's also different for every niche. For example in an easy niche, you can fix a page's SEO and a couple of days later it will move up the rankings 5 spots, say from position 12 to position 7. Then in a competitive niche you can spend 100 hours doing onpage SEO and the page position might improve by 1 spot over the course of 3 weeks etc. The best way to test different on-page factors is with single variable tests in isolation using fake keywords like the guys at SIA (Search Engine Intelligence Agency) do it. Then you pin down the most effective tweaks and bust a lot of myths as well.