How do Alexa and Compete get their data? which one is better?
Gathering Web Usage Information
In addition to the Alexa Crawl, which can tell us what is on the Web, Alexa utilizes web usage information, which tells us what is being seen on the web. This information comes from the community of Alexa Toolbar users. Each member of the community, in addition to getting a useful tool, is giving back. Simply by using the toolbar each member contributes valuable information about the web, how it is used, what is important and what is not. This information is returned to the community with improved Related Links, Traffic Rankings and more.
Compete’s clickstream data are collected from a 2,000,000 member panel of US Internet users (about a 1% sample), using diverse sources. Using a rigorous statistical normalization methodology, Compete creates precise projections of the behavior of the entire US Internet browser population on monthly and weekly basis. In addition, Compete provides daily estimates of share of consumer attention garnered by the top Internet sites and the velocity of change of this attention. Compete is the only commercial web analytics provider to make its data freely available online for all Internet users.
From my own experience, I think Alexa is more reliable, because its user database is bigger.
It's not really that easy to compare, because they deal in different metrics.
Looking at Compete's Unique Visitors for a site that i'm involved in, they give an average of 4,000 unique visitors per month. Google Analytics for the site average 180,000 unique visitors per month.
For Alexa, the metrics are all expressed as a percentage of global stats, or " unique pages per day" -- things that no one else uses, so there's no way to gauge the accuracy of the stats.
However, i believe that the problem is the small, US-centric sample which penalises non-US sites (FYI, the site i'm talking about here is for a business in South East Asia).
Moreover, looking at the textual information that Alexa presents, i'm concerned that it's several years out of date, which makes me doubt the efficiency of the system. According to Wikipedia:
On November 27, 2008, Amazon announced that Alexa Web Search was no longer accepting new customers, and the service would be deprecated or discontinued for existing customers on January 26, 2009
The Wikipedia article is a good read, as it talks about the accuracy of stats.
Final word: trust only verified stats!