The sample size for the study was 16,000 units, but there are some notable facts to note. Most of the Wii failures were due to power issues and most of the non-RROD errors for the Xbox 360 was "Disc Read errors," the same as the PS3. Usage was also a factor, because the Wii, according to Nielsen numbers, is just about half of that of the Xbox 360. The report says the average 360 console is played or used for 1191 minutes per month, followed by the PS3 at 1053 and the Wii lagging way behind at 516.
Perhaps more notably, the report concludes that the new "Jasper" based Xbox 360s, which began shipping in late 2008, have almost completely killed RROD, and SquareTrade expects the failure rate to continue to fall as 2009 continues.
You can read the full PDF here: SquareTrade console reliability PDF