When to use something
other than MTBF
As you may suspect I would say you should never use MTBF.
Given MTBF is prevalent, we may find avoiding MTBF nearly impossible.
Given a choice
When talking about reliability goals, just use reliability. Say what you mean in clear language. For example, if you want 95% of units to survive without failure for 5 years, than say the reliability goal is 95% survive over 5 years (include function and environment if it’s not clear from the context)
When asking for reliability information, as for what you want. If you want the device to last five years without failures or with very few failures, then just saying 5 years can be misunderstood. Couple the duration with the probability of survival, to be very clear.
When specifying a test, also be clear – the goal or objective is one statement, the confidence or statistical uncertainty elements is something different – keep them separate.
When not given a choice
When only given MTBF or only asked for MTBF values, what should you do. Well, use the value and ask some questions. Remember that MTBF all by itself is just an indication of the average failure rate. It is not a duration and does not convey how long or over which period of time the failure rate applies.
I cringe when I hear someone comment on a 50,000 hour MTBF value with, “That is about 5 years, which is long enough for our application.” We really should state MTBF as hours per failure to be a bit clearer.
So, when given 50,000 hours MTBF for an item, I first consider over what duration this applies (if I don’t know – it’s time to ask more questions). So, let’s say we have a electronics box with a fan. It is expected to operate full time for two years, or 17,520 hours of operation.
If the fan assembly data sheet has a listed MTBF of 50k hours, and it’s the only information I have available. I can estimate the reliability directly.
This is the reliability function for the exponential distribution and results in an estimated 70% of units survive over 2 years. If that is an acceptable failure rate (about 30%) then use the fan, if not, find a better fan, or a better estimate of the reliability of the fan.
When only given MTBF do the math and convert the value into something that is much easier to understand.
Do the same when asked for MTBF. Provide reliability – probability of success over a specific duration. Again, make it clear.