Peter Norvig, director of research at Google, has published some stats showing what many online marketers have been saying for years — Alexa data is skewed by the fact that more online marketers download the Alexa toolbar.
Alexa has come under fire repeatedly in recent years, from marketers and competitors.
Norvig’s limited test with five tech sites shows that Alexa creates as much as a 50:1 selection bias for certain sites, such as that of Matt Cutts, who discusses the stats on his blog. Because more search marketers tend to download Alexa’s toolbar, Cutts has an Alexa pageview ranking that’s about 25 times that of Norvig, though actual server stats show that Norvig’s site gets nearly twice the pageviews.
“What that means is that people with the Alexa toolbar installed are 25 times more likely to view a page on Matt’s site versus mine, but overall, all users view twice as many pages on my site,” he writes.