These are the stats for which browsers people are using. They're gathered from the page logs, and are based on hits (as opposed to pages, visits or sessions).
From Jan 2004 to Jan 2006, the groupings are based on Webalizer's merge level 4, with additional consolidation of MSIE 5.[0-4] as a single number.
Data is missing from Feb 2006 through May 2006.