What % off next-gen console owners actually own multiples? Bad science?

[theRepublic]

I found articles about the study on Gamasutra.com and GamersReports.com. However, I can't even find the actual press release on NPD's own website. There's no way that we will get actual data from the study from them. The most I found out about the study is this:

Methodology
Data was collected via an online survey of 20,240 NPD Consumer Panel members ages 2 and older. Respondents age 2-12 were captured via surrogate reporting whereby the female parent (age 21-55) brings her child to computer to answer questions, either with or without parent’s assistance. In order to qualify as an online gamer, respondents had to use one or more of the following systems to play games online: Nintendo DS/DS Lite handheld system, Nintendo Wii, PlayStation 2, PlayStation 3, Sony PSP handheld system, Xbox, Xbox 360 or PC. Gamers overall (whether online or offline) were included in the study as well. Fieldwork was conducted from January 11 – February 5, 2008.

I'm always a little wary of studies that rely on these voluntary surveys of consumer panel members.