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Hardware manufacturers often quote figures like 300,000 hours for the Mean Time Between Failures (MTBF) of a hard drive, but this is not a very useful measurement as it derives from tests on new drives. In fact, the probability of failure of computer hardware follows a U-shaped curve when plotted against time. New hardware has a period of relatively high "infant mortality" during which failures caused by actual manufacturing faults occur. Good manufacturers may perform a burn-in test by running their hardware for a while, so that the customer does not encounter early failures. After this early period, there is a long period of high reliability. Then there is a period of slowly rising failure rates caused by the hardware wearing out.

Sometimes manufacturers quote an "expected service life" for a piece of hardware, typically 3-5 years for a hard drive - this is a more useful figure than the MTBF. The warranty is also a useful guide, since the manufacturer is betting real money on most devices working for at least that long.

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