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I saw the PR of Stack Overflow and it is 7.
If PR is the rank given to the website for its quality, why is the PR of Stack Overflow just 7?

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PageRank is not an indication of a website's quality. It's an indication of a web page's link popularity. So StackOverflow does not have a PR of 7. It's home page has a PR of 7. If you view StackOverflow's inner pages you will see they have their own PR as they should since every page has PR. Those page's should have a lower PR then the home page as a well constructed website will naturally channel PR to the homepage through internal linking but it is not unheard of to have an inner page with a higher PR then a site's home page. This can happen if a website has a particularly popular article which has attracted a lot of incoming links especially high PR ones.

A PR of 7 is very good and I would say a reasonable PR for StackOverflow's home page to have.