Alexa is an impartial measurement service. Our data is generated automatically for the over 20 million websites that we measure, and our ranking methods remain constant. We still use a combination of unique visitors and pageviews in order to calculate ranks for sites.
However, over time the composition and size of our data panel does change. Our data panel is the sample of global internet traffic that is used to calculate Alexa ranks and estimate non-Certified metrics.
Changes in the data panel are a natural occurrence due to the evolution of internet audiences and usage, as well as improvements by Alexa to capture the resulting traffic data.
Alexa strives to calculate unbiased estimates of the actual web traffic to all sites on the internet. However, we do understand that biases exist and we frequently update our calculations to account for them. For more information about how biases can affect our estimates please see this page.