Any Liar Can Figure …

but figures don’t lie. Or so the saying goes.

Many people wonder why, since the figures state incredibly long times of service, why their hard drive has failed them. Hard drives these days have ratings of, when converted to years, around 150! How can the drive they bought have possibly failed them. Were they born under a bad sign? Made a voodoo priestess mad? Angered God in some small way?

What most people don’t understand, other than statistical methods, is exactly what the abbreviation MTBF explains.

First, the abbreviation stands for Mean Time Between Failures.  Second, for those mathematically-challenged mean is mathspeak for arithmetic average.

The upshot of the recent studies can be summarized this way: Users and vendors live in very different worlds when it comes to disk reliability and failure rates.

Consider that MTBF is a figure that’s reached through stress-testing and statistical extrapolation, Harris says. "When the vendor specs a 300,000-hour MTBF — which is common for consumer-level SATA drives — they’re saying that for a large population of drives, half will fail in the first 300,000 hours of operation," he says on his blog. "MTBF, therefore, says nothing about how long any particular drive will last." In other words, MTBF does a very poor job communicating what the actual failure profile looks like, he says.

It’s like providing the average woman’s height in the U.S. but without showing the numbers used to derive that average, Smith says. "MTBF became the standard because it was perceived as a simpler answer to the question of reliability than showing the data of how they arrived at it," Smith says. "It’s an honest-to-God simplification."

 

The above excerpt from ComputerWorld relates some of the problems when using terms that are not well understood, and why the average user has a jaundiced view of the quality of manufacture of these marvelous devices.

What would make a little more sense would be a clear explanation of MTBF on each retail box, and perhaps a bell curve showing at what other points in drive life the failures tend to cluster.

When reminded of the nature of moving machinery, and the quality of most drives, along with that explanation, the average user can approach a dreaded drive failure with the proper attitude and degree of acceptance.

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Tags: mtbf, hard drive failures, sata drives, statistics, perceived quality