> So... you design your product, set up a million of them, measure how long 
> it takes each one to fail, and take the arithmetic mean of that?

For our products at least, we usually set up 5-50 of them depending on what 
the final volumes will be.  Before mass production has started it is very 
expensive to make products, so you try to keep the numbers down as much as 
possible.

Usually we have a target from our customer, like 10000 hours, so we design 
with that in mind.  The testing usually confirms that everything will last 
ok for 10000 hours, but sometimes we need to go back to the customer and 
negotiate a change in spec if the results aren't so good.  Either they then 
tell us to fix it and we go through another 10000 hour test, or they agree 
with the spec change (usually accompanied with a price reduction!).

One cool thing (literally) is that before an entire car goes on sale it must 
go through similar tests, they essentially stick it in a freezer for 6 
months at -40 degrees and then check that everything works afterwards.  This 
is part of the reason why cars take several years to design and test before 
they are sold.

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