Alexa is actually a trustworthy index of the page’s popularity. However, the most important thing to remember about Alexa Traffic Rank is that it is a REVERSE indication. The LOWER the number is, the MORE hits the website gets. Figures from 10,000 and lower indicate really good traffic. So, Alexa Traffic Rank is simply a traffic-based indicator for the whole website.

On the other hand, Google PR shows how authoritative a particular web page is for Google. It's given to a web page, not to a website and is a combination of many SEO factors: domain age, domain thrust rank (shows its importance of it to Google), inbound links (their quality and quantity), etc.
Simply put, Alexa is for traffic, while PR is for authority.

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