How does Alexa decide sites traffic rank?

I’m using the Alexa tool as a good idea to get a rough gist of how much traffic a website gets, i’m looking to hoard some domains, specifically ones with a possitive alexa taffic rank, ie not something which says ‘no data’. If something is ranked 7,029,039 lets say, how many visits would site/domain get daily? or does that include Google searches too? I know this is a broad question but i’m curious.

They have access to ISP data that they use to make this determination in addition to their toolbar. It is far from accurate and certain practices like an image from a domain appearing on many sites can make the rank seem artificially high.

Alexa is not a tool that will give accurate data, it is differentiate traffic from ISP data and people using their toolbar and accessing your website. I would recommend reviewing for competitor analysis.

Alexa metric collect data from million of browser toolbar, when more user install Alexa toolbar the data will become more accurate. Personally I only take Alexa rank below 50K as a gist of traffic estimation.

[FONT=verdana]Pesonally, I would only use Alexa to indicate a comparison between sites - not any actual site-specific data. For example, if it ranks Site A at 7,029,039 and Site B at 14,087,056, then it seems likely that Site A will get roughly twice as much traffic as Site B. But even that can only be an approximation.

The only people who know for sure how much traffic an arbitrary site gets (or where that traffic comes from) is the operators and hosting company of the site itself. Even Google doesn’t have that information. Alexa can give you a very rough guide to a site’s performance, but no more than that.