Matt Cutts has come out with his second post on Web Metrics - looking at this favorite case, Alexa.  Matt seems to focus on Alexa because it’s an easy target that is more wrong than right.

One possible source of skewing in Alexa data is a bias toward webmaster-y sites. Alexa shows how popular web sites are, so it’s natural that webmasters install the Alexa toolbar. Some do it just so that their normal day-to-day visits around the web (including their own site) are added to Alexa’s stats. The net effect is that webmaster-related sites are going to look more important to Alexa. Let’s take a look at a graph comparing mattcutts.com and ask.com:

For now, let’s concentrate on the green ellipse. This is a graph of reach, which is defined as “out of one million internet users, how many of them went to mattcutts.com vs. Ask each day.” If you look at the green ellipse, it shows that I had a spike in May and Ask had a dip in June. I believe Alexa was reporting that for at least a good day for me and a bad day for Ask, I was reaching more internet users as a percentage than Ask. (Alexa folks, please correct me if I’m mis-speaking or drawing the wrong conclusion.) And I believe that I can safely say that’s not remotely close to true. I have nowhere near the reach that Ask has.

Most of the tools we can use for free have data that is not that precise - you can’t take any of it to the bank - you just use it to support a opinion you already have as additional evidence - and that’s about all it can be at this time.