Go to InformationWeek.com, ComputerWorld.com, and ITWeek.com, or pick a few other trade rags that publish a media kit. Download the media kits, see how much tr

Go to InformationWeek.com, ComputerWorld.com, and ITWeek.com, or pick a few other trade rags that publish a media kit. Download the media kits, see how much traffic these similar sites get, and then see what the error is on Alexa. I think you'll find that the numbers are of the right order of magnitude. You're going to get a pretty clear signal when looking at sites relative to one another, since most of the error will effect the sites the same. And if you have sites that have the same sorts of traffic patterns, then the relative signal should be even clearer. It's true that people gaming the system effect the numbers, but that's not going to distort the numbers enough to make them worthless. You can't use Alexa or similar to see whether one site has 3000 visitors a month and another has 3500. But if one ha 3000 and one has 30000 then you'll see it in the graph.