The usage share of web browsers is the portion, often expressed as a percentage, of visitors to a group of web sites that use a particular web browser.
Accuracy
Measuring browser usage in the number of requests (page hits) made by each user agent can be misleading.
Overestimation
Not all requests are generated by a user, as a user agent can make requests at regular time intervals without user input. In this case, the user's activity might be overestimated. Some examples:
Certain anti-virus products fake their user agent string to appear to be popular browsers. This is done to trick attack sites that might display clean content to the scanner, but not to the browser. The Register reported in June 2008 that traffic from AVG Linkscanner, using an IE6 user agent string, outstripped human link clicks by nearly 10 to 1.
A user who revisits a site shortly after changing or upgrading browsers may be double-counted under some methods; overall numbers at the time of a new version's release may be skewed.
Occasionally websites...