In the first 10 days of April, Windows 7's average daily share was 32.2 percent as measured by StatCounter, besting XP's average of 30.7 percent.
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It was the first time that Windows 7's U.S. usage share was higher than Windows XP's.
Microsoft has touted Windows 7's sales numerous times since its October 2009 debut, and has repeatedly said the operating system is its fastest-selling ever. Analysts have agreed that Windows 7 has been a success, especially compared to the lukewarm reception customers gave its predecessor, Vista.
Last January, Microsoft announced it had sold over 300 million Windows 7 licenses.
By StatCounter's tracking, other widely used operating systems in the U.S. include Windows Vista, with an April average of 19.5 percent, and Apple's Mac OS, with 14.8 percent.
Vista peaked in the U.S. at 35.6 percent in August 2009, two months before Windows 7's launch, while XP first fell under 50 percent at the same time.
StatCounter's global numbers are different, showing that U.S. users have flocked to Windows 7 faster than the average. Windows 7 accounts for 31.5 percent of all operating systems in use around the world so far this month, while XP remains the most-widely-used OS with a 46.8 percent share.
Data from rival metrics vendor Net Applications paints a less-impressive picture for Windows 7. Net Application's March numbers put Windows 7's global usage share at 24.2 percent, compared to XP's 54.4 percent.
Both Net Applications and StatCounter calculate operating system usage share by monitoring the machines that reach their clients' sites. The former, however, also weights its numbers by country.
Like most Web measurement firms, Net Applications has more data on some countries -- the U.S., for instance -- but relatively small samples from others, like China. So to produce what it believes are more accurate numbers, Net Applications weights its Chinese data proportionally higher because that country has a greater percentage of the world's Internet users than the U.S.
The result: Because China accounts for a major chunk of all computer users and because a higher percentage of Chinese users run Windows XP than those in virtually every other country, Net Applications' XP numbers are higher than StatCounter's.
Although Microsoft has not told users to dump Windows XP -- a recommendation it's made about the 10-year-old Internet Explorer 6 (IE6) -- the company has urged customers to leave the operating system behind. Microsoft executives, for example. have not been hesitant to call XP "the lowest common denominator" as they push IE9, the new browser that only runs on Vista and Windows 7.
According to StatCounter, people in the U.S., at least, are listening.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed. His email address is email@example.com.
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