How do you talk about reliability?

How do you talk about reliability?

How do your customers? And, your suppliers?

If they are all using MTBF, there is a pretty good chance one or more parties involved have some misunderstanding of MTBF. It’s common for some to think it is a failure free period. Or they don’t count early life or wear-out type failures when doing the calculation.

Product Failure

A product failure is a failure and they all count to the customer who has lost the functionality they expected.

The last time I ran across someone asking for MTBF I asked them what they meant or really wanted. They wanted no or few failures over two years. That was good enough. So, I asked them if 98% surviving over the two years was good enough. It was.

It was a very short discussion.

We both knew that having less than 2% fail over two years was only 2 out every 100 units would failure over the two-year period. No assumptions. No misunderstanding.

Sure, we would need to define ‘failure,’ yet I maintain that if the product doesn’t meet the customer expectations, doesn’t work, works poorly, whatever, it is a failure.

The customer defines failure whether we agree with them or not.

Full reliability specification

Most reliability treaties define reliability at some point as including four elements.

  1. Function – what the product is supposed to do.
  2. Environment – where and under what conditions.
  3. Probability – how many will survive without failure
  4. Duration – over what period of time or cycles.

It is really simple.


My reliability specification recommendations are

  1. Ask what they mean — get a full reliability specification.
  2. Find out what is a failure for the customer?
  3. Use a percentage surviving over a specific time frame. Be clear.

And, by all means, as soon as someone mentions MTBF – ask they what they mean by that term.

About Fred Schenkelberg

I am an experienced reliability engineering and management consultant with my firm FMS Reliability. My passion is working with teams to create cost-effective reliability programs that solve problems, create durable and reliable products, increase customer satisfaction, and reduce warranty costs.

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