Alexa tracks traffic using a cookie that is routinely removed by anti-spyware programs.
It may not be accurate, but might not be completely useless. Alexa can be used as a general metric to see how much traffic your competitors are getting relative to Alexa's flawed estimate of your own traffic.
If you want to see how well your own sites are doing, use your sever logs. Google Analytics is also not accurate because it cannot track data for anyone who has JavaScript disabled. That can be up to 5% of your visitors. When an IE user sets the Security Level to High, JavaScript is disabled.
You can have it fast, good or cheap. Pick any two.
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