on April 22nd, 2014

Dear Colleagues,

As you all know – reliability is a highly sought after (especially in engineering) measure of how long a product or system functions correctly. The ultimate goal is always a reliability of 1 or 100% reliability with no failure over the life of the product. Reliability of 0 refers to immediate failure and should obviously be avoided.

You may recall the old ‘bathtub curve’ of reliability. At the beginning of a product’s life there is a heightened probability of failure. Subsequently, during the product’s life time this drops to a hopefully very much lower probability. Near the end of the product’s lifetime (as it wears out); there is again a heightened probability of failure (as you would expect). Hence the reference to a bathtub curve.

I am sure you have become irritable when a product fails soon after purchase. Or immediately after the warranty period has expired (e.g. phones).  Leading many cynics to refer to the vendor’s (often unethical?) ‘planned obsolescence’ of a product where it fails after a given planned time. It fails so that you are forced to upgrade or buy a new one.

100% is Crucial for Safety
The target reliability of a pacemaker has to be 1 because failure may result in loss of a life. Similarly with a bridge or other critical elements in our lives.

Cost Plays a Key Role in Reliability
However, most consumer products (e.g. a toy, radio or dishwasher) most decidedly do not have a reliability of 1. Designing for perfect reliability would result in a very expensive product resulting in loss of competitive advantage to the manufacturer.

Oddly enough, some aircraft parts are not 100% reliable as weight has to be minimised.  This shortcoming is dealt with by a rigorous inspection regime (e.g. for cracks) and regular replacements of parts. The growth in (often illegal) second hand defective parts for aircraft is thus of major concern as their reliability would be unpredictable and certainly not what the manufacturer originally anticipated.

An interesting take by Orison Swett Mard on success: Success is not measured by what you accomplish, but by the opposition you have encountered, and the courage with which you have maintained the struggle against overwhelming odds.

Thanks to 101 Things I learned in Engineering School by John Kuprenas with Matthew Frederick.

Yours in engineering learning,


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