The Alexa Blog: Alexa 101: The Anatomy of the Traffic Rank Graph

The Alexa Blog: Alexa 101: The Anatomy of the Traffic Rank Graph: "The main Alexa traffic rank (as displayed in the Alexa toolbar) is a three-month rolling average, while the points in the Traffic Stats rank graph are daily. While the daily Alexa Traffic Ranks allow you to see fluctuations in a site's traffic on a very short timescale, the longer-term averages listed on the site information pages (and used for our Top Sites lists) are much more robust and authoritative for sites with low traffic."