How Accurate is Alexa? The Stats Don’t Lie
April 14, 2010 23 Comments
After gathering monthly stats on 40+ blogs and comparing their online ranking by various tools (Alexa, google reader, Quantcast, etc) to actual page views, I wanted to take a few posts to put the data out there on how accurately some of these online tools actually rank sites. Alexa is a site that is designed to rank websites based on traffic. An alexa rank of 1 would be the most visited site in the world (google, facebook is #2). A rank of 1 million would predict that website to be the 1 millionth most visited site on the web. So the higher your rank, the less traffic you would get. That is the theory.
One of the problems with alexa is we don’t often have the actual numbers to compare ourselves to in order to see how accurate alexa actually is. Well, now we do have some data. After requesting the data from 40+ blogs we have enough data to run some Pearson correlations to see how well alexa ranks blogs. Alexa does claim to be less accurate on blogs with higher rankings so we have to take these numbers with a grain of salt. However, the average user of alexa does not have a top 100,000 blog, much less a top 50 blog. So are the numbers from alexa practical at all?
First, there is the correlation on the entire data set. There were 32 blogs that had alexa ranks (between 139,913-19,740,920). Alexa rank was correlated with those 32 blogs actual page views as submitted by each blogger. Remember, the lower your rank, the higher you would expect your page views to be. That means you are expecting this correlation to be negative and statistically significant (2-tailed significance less than .05). A correlation was run, a Pearson R (2-tailed) with SPSS. It showed there was not a significant correlation between actual page views on alexa rank. R = -.286 (2-tailed significance of .113). In other words, having a low alexa rank does not actually correlate with having a higher number of total page views in a given month. At least, that seems to be the case on blogs ranked in the Alexa range of this data set as defined above.
Is alexa actually more accurate the lower your rank? In order to test alexa’s own claim that it is more accurate on lower (more trafficked) ranked blogs the data set was split nearly in half with basically half the blogs falling below an alexa rank of 3m (18 blogs) and half above (14 blogs). As alexa predicted, the lower ranked blogs did have a higher correlation R = -.445 (2-tailed significance of .064). That is extremely close to being statistically significant and is in the expected negative direction. The higher scoring blogs did not do as well and actually had a positive correlation. That is not what you want, as a lower alexa rank should translated into more page views. R = .238 (2-tailed significant = .412). So alexa’s claim that it gets more accurate the lower the rank gets appears to be true.
Bottom line, alexa does improve as your blog gets more traffic but it is still far from perfect. None of these correlations were statistically significant. The closest was on blogs with more traffic. It would be interesting to gather data on blogs with alexa ranks under 100k and have 50-100 blogs in the data set to see what the results would show.