The reported accident rate for iPhone 4s, which includes more than just broken screens, was still an overall 68 percent higher than for the iPhone 3GS. This number is calculated by comparing users reporting an accident: 4.7 percent of iPhone 4 owners to just 2.8 percent of iPhone 3GS owners. Disregarding the four month timeframe, iPhone 3GS is actually at a 7.8 percent accident rate, while iPhone 4's accident rate is projected to reach 15.5 percent.
SquareTrade says that this is due to two factors: the scratchable surface area has doubled while the new aluminosilicate Gorilla glass used in the iPhone 4 doesn't seem any less likely to break than in previous models. It's also worth noting that the company "considers any claim involving physical damage to be categorized as 'accidental damage,' and in our initial comparison we only look at accidental damage. Some owners choose not to report minor damage, which means that actual accident rates may be even higher than what is shown."
The report follows news that the iPhone 4 has a design problem with shattering glass backs resulting from particles scratching the back of the phone while taking a non-bumper case on and off. SquareTrade says its data doesn't identify which broken screens resulted from dirt trapped behind a case, but it did note that at least a quarter of the broken glass claims involved the back screen.