As my colleague , rival measurement firm NetMarketShare comes up with wildly different browser share data for the first week of May. StatCounter's data shows 32.76 percent share for Chrome and 31.94 percent for IE, while NetMarketShare shows 54.01 percent share for IE and 18.85 percent for Chrome.

According to NetMarketShare, Chrome hasn't even , though StatCounter says Google achieved that milestone late last year.

How can two measurements be so different? The answer comes down to two major differences in methodology.

Views vs. Visits

Both firms base measurements on traffic data from a network of sites--40,000 sites for NetApplications and 3 million for StatCounter. NetApplications measures unique visits, so if you go the same Website twice, you'll still only be counted once per day for market share. StatCounter measures page views, so a user may be counted several times per day on the same Website.