Once installed, the toolbar collects data on browsing behavior which is transmitted to the website where it is stored and analyzed and is the basis for the company's web traffic reporting.
There is some controversy over how representative Alexa's user base is of typical Internet behavior. If Alexa's user base is a fair statistical sample of the Internet user population . Alexa's ranking should be quite accurate. In reality, not much is known about the sample and possible sampling biases. Alexa itself notes several examples. A known source of bias is the self-selecting, opt-in nature of Alexa traffic tracking software installation, but the significance of this bias on rankings is not reported.