According to a new study by the NPD research group (http://www.npd.com), one in four iPad purchasers say it's their first Apple product. The study also shows that there's lots of room for growth for Apple products.

NPD's "Apple Ecosystem Study" found that just 33% of U.S. homes own Apple products. But that number is growing well past the 37 million American households Apple devices are currently found in, thanks in large part to the iPad.

"iPad sales are growing much faster than any other Apple product has this soon after launch," says Ben Arnold, director of industry analysis at NPD. "In fact, one-in-five Apple owner households has one -- nearly equivalent to the number that own an Apple computer. This demonstrates the appeal of both the new form factor and Apple’s app ecosystem."

A majority of the U.S. households that do have Apple products (67%) own an iPod, and 82% of iPod owners polled said the media player was their first Apple product. But as the popularity of the iPod has declined, Apple's new "halo effect," driving sales of other devices in its ecosystem, is being led by the iPad and iPhone.

"While over 70% of long-standing Apple owners began their relationship with the brand by way of the iPod, this number declines to just 57% among those entering the Apple franchise in the past two years," the NPD Group said. "Newcomers to the brand increasingly turn to the iPhone or iPad as their first Apple device, which combined account for one-third of first-time Apple purchases since 2010.

The average home with Apple products owns 2.4 devices built by the company, but most "Apple households" aren't strictly tied to the Cupertino, Calif., company's products. Six in ten homes with a Mac also own a Windows PC, and nearly 30% of so-called Apple "enthusiasts" own a smartphone other than the iPhone.

These numbers show the incredible growth potential Apple still has in the U.S., as two-thirds of American homes don't any products by the company, and nearly a third of those in the ecosystem don't own an iPhone. So look for Apple sales and stock to keep going up, up, up.

(The data comes from a survey of more than 3,000 consumers conducted by the NPD Group in February. It was balanced to represent U.S. household demographics.)