Wednesday, December 10, 2014

Tale of Two Curves

You're probably familiar with the Bell Curve, a "graph of normal distrubution;" it's a good, simple tool, a template that seems to be useful in descriptions of reality.  If it's a graph about people, for example, that 70% around the middle (mean) line would be most of us, who muddle along and don't do too much harm, while the 30% at the ends would be the sociopaths, psychopaths and severly disabled.  Since the 1994 publication of the book of the same name, it's been largely associated with the distribution of human intelligence, which the authors posit is a better predictor of outcomes in life than environmental factors.  There was a Three Stooges short film, without graphs, which explored the same point without the subsequent decades of controversy.

I recently came across the above, known as the Bathtub or Weibull Curve.  It is used to display predictions of product reliability in engineering.  Hmmm -- I wonder where the Vega would have been placed on such a chart by GM engineers, back in the day?  Manufactured products are most likely to fail at the very beginning (labelled "infant stage" here) or at the end of their service life, but they may be obsolete before they age out; either way their service life is over.  Think of cell phones or all that video equipment bought for Christmas or vacations over the years that was quickly discarded as lighter, more advanced products were introduced, despite their still being in good working condition.

Another major factor in determining the end of service life is the repair expense compared to the value left.  I took a small trimming router to a local shop years ago to have the motor rewound after years of hard use; the estimate was $100, five dollars more than what I had paid for it new.  A new one it is, then.  If a part is inexpensive (and most everything is easily available on the internet now) and you can replace it yourself, like a broken belt on a cassette player, that's much better than either a repair bill or a replacement purchase.

Cars now serve reliably for about ten years or up to 150,000 miles, which certainly was not the situation when I was younger.  People traded in at three years back in the 50s, 60s and 70s because that's when things rusted, leaked or just snapped.  That is still practiced, with auto leasing programs (financial suicide is painless, the salesman assures you), but the practical reason for doing so is long gone.

Clustered on the left side of the graph you'll find any Chinese-made product that uses electricity.  There are companies, American and German, that have made reliable switches for electrical devices great and small for a century (Allen-Bradley, Crouse Hinds, Bosch, etc.); you would think Chinese manufacturers could copy some of that tried-and-true technology like they do everything else, but I and millions of others can tell you they can't make any kind of a switch that will last.  Usually they fail at 367 days on a product with a one-year warranty.

This will all be on the test.


1 comment:

  1. 'cuse me professor, but I was out that day. And my dog ate the homework.