Giving a presentation last week and asked if anyone uses an 85/85 type test, and a couple indicated they did. I then asked why?
The response was – just because. We have always done it, or it’s a standard, or customers expected it. The most honest response was ‘I don’t know’.
They why is the test being done? Who is using the information for a decision? What is the value of the test results? If ‘just because’ is the best you can say about a test, why do it?
The same applies to MTBF. Why is it being used and for what purpose and with what value? If the response you find is basically, ‘just because.’ Stop using MTBF!
The basic question that then arises is what should we use instead. The answer is or should be obvious – what matters to your customer and your business. If you customer wants uptime – use availability. If you customer wants durability, then use reliability.
Reliability is the probability of successfully operating over a stated period. As you may know from my previous posts, some confuse MTBF as meaning the same thing. And, as you know, MTBF is a statement about the failure rate, and not a couplet of probability and time. It only have half of what’s needed.
Use Reliability. State the probability or percent that survive and state the period. 98% survive one year. Easy.
No assumptions about distributions or statistics. No simplifications or distortions. And, it’s straight forward to understand. It means what it means. 98 out of 100 units operate successfully for one year. Easy.
Based on this metric, we can determine or assume life distributions and answer all manner of queries. It’s just a start, yet directly useful and meaningful.
Why? Not just because. Reliability is a measure of what the customer or business needs. It directly relates the number of units that work over a period of time. For example, if we have a one year warranty period and want about 2% or fewer failures during the warranty period. Then saying 98% reliable over one year (a bit more positive statement then 2% failures) works just fine.
Sure this could be converted to MTBF – and again I would ask why?
The Reliability Metric Book Announcement (book)
Considering WIIFT When Reporting Reliability (article)
What Does ‘Lifetime’ as a Metric Mean (article)
How to Translate Customer Expectations About Reliability (article)
Bjarni Ellert Ísleifsson, CMRP says
Thank you very much for the like on my post : http://bjarniis.wordpress.com/2012/01/08/logic-will-get-you-from-a-to-b-imagination-will-take-you-everywhere-albert-einstein/
I would like to say I like that you would like to focus on positives rather than negatives. Uptime (reliability) is positive MTBF is a negative measure.
However these are almost inverted measures that would almost mean the same if presented correctly… For example, we only have 2% MTBF, or we have a impressive 98% reliability!
I would definitely select tha latter to present to my customers.
Thank you again for your contribution to the great world of reliability. 🙂
Greg McLean says
I recently found my way back to Reliability (I went over to the dark Quality side for a dozen years) and I was surprised to see how little it has changed. This is a revolution that is worth fighting for!
I have customers who demand to see the MTBF, then they want me to put the early life failures back into the equation because they need to have the “complete picture” … then they ask why did the MTBF go up??? (even with a mixed Weibull model)
I am a huge advocate of stating the reliability at a specific time. As you say, it is positive, but I find the best trait is that it is easy for the audience (once they catch on). It tells them how many will survive in a time period that matters (e.g., warranty) and it is easy to put multiple pieces together to get a system reliability (and it doesn’t matter how the different failures are distributed!)
Fred Schenkelberg says
Hi Greg and welcome back to the Reliability side of things.
Thanks for the comments and I agree, helping to translate reliability into number of returns during warranty (or better the cost of warranty) is sound advice.
Let us know of any local victories with the eradication of MTBF. And, of course, any ideas that may support the cause.
Hilaire Perera says
Although useful to some degree, the mean life function ( often denoted as MTTF or MTBF ) is not a good measurement when used as the sole reliability metric. Instead, the specification of a reliability value with an associated time, along with an associated confidence level, is a more versatile and powerful metric for describing product reliability