[time-nuts] MTBF (was Rubidium standard)

[Mike S]

At 04:21 PM 11/18/2009, Alan Melia wrote...
>Sorry Mike , unless, as someone else said, the figures are derived 
>from
>field failures over at least a good porton of the expected like the 
>MTBF
>tells you absolutely nothing!!

That is exactly what I meant by "the real-world statistical form" - 
data gathered from in-service operation, not lab tests (or worse, a 
purely mathematical combination of individual component MTBFs, like one 
resistor contributes X, an IC of a certain type contributes Y, etc.).

But, things have to start from somewhere (e.g. a need to deploy spares 
for a new product) so manufacturers will use shortcuts (lab testing, 
etc.) for initial numbers, then refine based on experience. That 
doesn't mean the shortcut methods are useless, only that they're less 
accurate. Something is better than "nothing," and a simple device can 
be expected to have a higher MTBF than a complex one (assuming similar 
technologies, manufacturing processes, etc.). So I don't agree with 
your "absolutely nothing" statement. Having an HP5071 doesn't make a 
Timex worthless.