Apple's Mac boosted its presence in American households by 33% in the last year, a research company said yesterday.

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According to a survey conducted by the NPD Group, about 12% of U.S. computer-owning households have a Mac, up a third from 2008, when the firm's polling showed 9% of the country's computer-equipped homes had an Apple system.

Households with Macs are also more than twice as likely as Windows PC-only homes to have three or more computers, said Stephen Baker, an analyst with NPD Group.

But what struck Baker was the number of Mac-owning households that also had at least one Windows PC in the place: Nearly 85% of the homes with a Mac also have at least one machine powered by Microsoft, Apple's OS rival.

"The percentage of households owning both Windows PCs and Macs has been increasing faster than Mac-only, faster than Windows-only," said Baker. "That was a little surprising," he admitted.

For some, he said, it might be more than that. "If you continue to believe that Apple just doesn't work with anything but Apple, you might be shocked. But when you think about it, the primary distinction Apple has now is its Mac OS. There's nothing really different in a Mac that you can't get anywhere else," Baker said, ticking off a list that started with the Intel processors found in Apple's computers, through USB and one to ATI or Nvidia graphics chipsets.

The NPD analyst might get an argument out of many that Apple's operating system is as much a distinguishing factor as it once was. Computerworld reviewer Preston Gralla, for example, recently pitted Apple's Snow Leopard against Microsoft's upcoming Windows 7, and essentially called it a draw .

Baker did acknowledge that some of the increase in Mac ownership, and the high rate of PC-and-Mac mingling, may be a result of Windows users switching to Macs, a phenomenon both analysts and Apple have used to explain Apple's sales surge in 2007 and 2008, when Microsoft's Windows Vista was often panned and sometimes reviled. "Certainly there's some of that, but you have to define 'switching'," said Baker. I think what we're seeing is switching from just a Windows household to a mixed household. There's not a lot of people moving to Macs and getting rid of their Windows PCs. Instead they're bringing Macs into the household.

"Overall, people are bringing more and more computers into their homes," Baker said.

Other tidbits from NPD's survey of more than 2,300 people included the unsurprising fact that households with Macs owned twice as many consumer electronics devices -- iPods, digital camera, GPS navigation devices and the like -- than homes with Windows-only computers.