Statistics

Alan Frayer, CNE, CNI, CIW CI, Net+, MCP

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I look forward to my mail delivery. Part of it might be the interesting tidbits I receive, allowing me to write this column. Then again, it might be the woman delivering the mail (this could also explain why Bocona always wants to be home when the mail is delivered). I'm happily married, still I'm not blind, and I have a certain degree of appreciation for her specifications.

A few long seconds after the mail is deposited in my hands, Bocona clears her throat and I turn my eyes to the treasure left behind. An electric bill (I swear the computer didn't use all that power!), some coupons, some press releases, some advertisements.

First stop, the trash, where the electric bill sometimes accidentally falls, along with a fair number of coupons. Next, I browse the ads; I can often find a few winners in there.

On computer components, my eyes are drawn toward specifications, and today I find a truly outstanding specification on a token ring card made by Hewlett Packard. Even HP finds it noteworthy, and spends a few precious inches expounding on this statistic: mean time between failures (MTBF).

This statistic is usually used to show reliability; the larger the number, the longer you can expect the item to last before requiring service. Taken literally, one can expect a large number of these components to be tested over time. When a part fails, they make note of how long it was in service, and at the end of the study they average those times together (calculating the mean), resulting in the MTBF.

With this in mind, prepare yourself for a miracle. This particular HP token ring card has a MTBF of 300 years. The thought staggers the mind. A proper test of this card requires a large number of them to be placed in service by the Pilgrims at Plymouth Rock. Hopefully most of these cards failed at nearly the same time, otherwise, for each failure before 300 years, there has to be a failure an equal amount later. If a card was DOA from the factory, then the last card to fail had to be placed in service before Christopher Columbus sailed.

I knew Hewlett Packard had a long history in electronics, but I didn't realize they'd been in business that long!

It is quite apparent that MTBF isn't as accurately calculated as suggested by the term. I'd be curious to learn just how HP arrived at the value. Until then, I might think twice about how useful MTBF ratings can be in rating reliability.

I smile at the revelation, and Bocona thinks I'm thinking of something (or someone) else. "Men!" she declares, storming back into the house. I watch her go in, wondering what it is I've done this time.


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