Try quantcast.com and compare the results, from what I have seen they are more accurate than Alexa anyway. The downside is that smaller sites are more likely to show up in Alexa than with quantcast.
I think that Alexa is one of the most important factor at the view point of traffic. so we are supposed to do work on Alexa even also.
Alexa can just measure bandwidth not actual traffic, which is why their rankings can vary so widely. I have sites with lower alexa ranking with lower traffic than other sites. It's just a minor tool in a toolbox. Check out Compete rankings if you're looking to cross reference. -Safe Travels