A Web measurement firm Wednesday blamed a massive online marketing campaign aimed only at nternet Explorer (IE) users for skewing its April data, which said Mozilla's Firefox and Apple's Safari had lost significant amounts of market share.

The company, Net Applications of California, has posted revised data on its Web site that shows Firefox and Safari still dropping in share, but by much smaller increments.

An "extremely large" marketing campaign among a small number of sites that targeted only users of IE was responsible for the aberrant numbers, said Vince Vizzaccaro, the executive vice president of marketing at Net Applications.

The additional IE traffic skewed the month's results in the Microsoft browser's favor; at the same time, it depressed the market share of Firefox, Safari and Opera Software's flagship browser, Opera.

Vizzaccaro would not name the company that owns the sites that ran the IE-only marketing campaign, but did say that it was "totally unrelated to anything with Vista, Microsoft or Windows."

Although Net Applications has seen a few cases where people have tried to "game" its system, Vizzaccaro doubted that was the case this time.

Net Applications analyzes traffic to approximately 40,000 business Web sites that it monitors for clients. It then collects and publishes some of the data, including which browsers are used to reach those sites and what operating systems users are running. It had filters in place, Vizzaccaro said, to alert Net Applications of anomalous traffic at any one site, but because the campaign was run among several, the company posted its data without investigating.

"This was very odd, and because it was spread out we didn't recognize the problem," said Vizzaccaro. Net Applications has since added new filters that should detect similar irregularities in the future.

Last week, when Net Applications posted its initial April numbers, Computerworld reported that they showed both Firefox and Safari had lost market share compared to March. Firefox, for instance, slipped to 17 percent, off nearly a percentage point from the 17.8 percent, of the month prior. Safari declined as well, according to Net Applications; in April, the company pegged the Apple browser's share at 5.5 percent,, off from March's 5.8 percent,.

IE, meanwhile, climbed to 76 percent, from 74.8 percent, the month before, said Net Applications last week, marking the first time since October 2007 that IE gained ground, and the largest single-month jump by the browser since Net Applications began posting its data in May 2006.

Those numbers were all wrong.

Net Applications' revised April data indicates that Firefox's market share drop was only 0.07 percent. Safari slipped too, but by just 0.01 percent. On the other hand, Internet Explorer's growth was really just 0.03 percent.

Vizzaccaro said that Net Applications had been taking a second look at the data when it was contacted by Mozilla, the company that develops the open-source Firefox. "We had a call from Mozilla, and working with them is what led us down this path to identifying that it was a very large traffic increase that was very specific to a group of sites," he said.

On Monday, Ken Kovash, Mozilla's metrics chief, posted an entry to his blog questioning Firefox's drop, and noting that Mozilla had contacted Net Applications.

"We had a very open dialog," said Vizzaccaro, who added that Mozilla shared its usage data with Net Applications. "Originally, we had a couple of theories, one of which was that this was related to Vista and SP1," he added. On April 23, Microsoft began pushing SP1 to users who had set Windows Update to automatically download and install the upgrade.

"The timing of this got us thinking of causes related to Vista SP1," Vizzaccaro said. "We thought the data supported that theory until Mozilla's numbers showed that Firefox's market share hadn't really changed."

Net Application amended browser market share data can be viewed on its Web site.