Apple's line of laptops ranked fourth in a multi-year reliability survey of nine notebook makers, according to a study of 30,000 portable computers published today by a company that provides after-sale warranties.

SquareTrade, which broke out the warranty claims of its customers by manufacturers, said that Apple took the No. 4 spot, behind ASUS, Toshiba and Sony, which held No. 1 through No. 3, respectively.

Over a two-year period, slightly more than 10% of Apple laptops -- the company sells two lines, MacBook and MacBook Pro -- failed in some fashion, said SquareTrade. The projected failure rate of Apple's notebooks within three years, added SquareTrade, was 17.4%.

ASUS, Toshiba and Sony, on the other hand, sported projected three-year malfunction rates of 15.6%, 15.7% and 16.8%.

"It's not really surprising that Apple's in the middle of the pack," said Vince Tseng, the vice president of marketing at SquareTrade. "What was surprising was that ASUS came out on top."

Tseng defended his company's rankings against the inevitable backlash by opinionated Mac owners. "Ours is pretty similar to other studies that have been published," he said, pointing to those done by Consumer Reports in particular.

Hewlett-Packard, which shipped more notebooks in the past year than any other OEM, came in dead last out of the nine manufacturers, with a two-year failure rate over 15% and a three-year projected failure rate of 25.6%.

SquareTrade based each company's three-year projected rate on the failure curve of all notebooks, which rises quickly from year one to year two to year three. "There is a notable acceleration of malfunctions in the second and third years," SquareTrade said in today's report.

While only 4.7% of all notebooks failed from a hardware malfunction in the first year of ownership, that more than doubled to 12.7% by the end of year two, and then leaped again to 20.4% by the time three years had passed.

SquareTrade said that the increasingly high failure rate was no surprise. "Laptops have a high usage rate," said Tseng. "People leave them on all the time, and notebook components are sensitive to heat. Two, they're portable and take a lot of abuse. And three, they're more complex than most other consumer electronics devices."

Apple may have played its cards right by not moving into the netbook market, according to SquareTrade's data. Netbooks, which the warranty firm defined as notebooks sold for less than $400, fail at a higher rate than more expensive alternatives. After just a year, netbooks fail at a rate 23% higher than entry-level laptops, those priced between $400 and $1,000, and at a rate 38% higher than notebooks that cost more than $1,000. By the end of three years, a quarter of netbooks will have malfunctioned, said SquareTrade.

"It's difficult to compare apples to apples," noted Tseng, "no pun intended," referring to comparisons between netbooks and higher-priced notebooks. "Netbooks have been sold in scale only the last 12 months, and up until the last six months, netbook sales were dominated by ASUS and Acer."

While ASUS held the No. 1 spot of the nine notebook makers, netbook rival Acer was No 7, with a three-year projected malfunction rate of 23.3%.

"It's just a guess, but one explanation for the higher failure rate of netbooks is that the components are typically cheaper," Tseng said when asked why netbooks failed at a higher rate than more expensive notebooks. "Early on, when component costs were fairly high [for netbook makers] because of their low volume, they were taking the lowest bidder on components."

Apple's executives, including its CEO Steve Jobs, have repeatedly dismissed netbooks when asked whether the company would enter that category. In October 2008, Jobs famously said , "We don't know how to make a $500 computer that's not a piece of junk."

SquareTrade's data was based on an average of 3,000 laptops from each of the nine manufacturers, and tracked the actual lifecycle of the machine, since it offers warranties only to buyers within 90 days of a hardware purchase. "The vast majority of the warranties are bought within 30 days," Tseng confirmed.

SquareTrade's report can be downloaded from the company's site ( download PDF ).