Some websites implement on-site analytics and publish the results of website ranking tools like Alexa. For such sites estimated metrics based on traffic patterns are shown across the web as a whole. Ranking tools identify those patterns by looking at the activity of millions of web users throughout the world, and using data normalization to correct for any biases.
The more traffic a site gets, the more data have to calculate estimated metrics. Estimates are more reliable the closer a site is to being ranked number 1. Global traffic ranks of 100,000+ are subject to large fluctuations and should be considered rough estimates.
If a site has Certified Metrics instead of estimated, that means its owner has installed code allowing us to directly measure their traffic. These metrics have a greater level of accuracy, no matter what the ranking. This way Alexa estimate site metrics.
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