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?
What Does ‘Lifetime’ as a Metric Mean (article)