I've been through this one.
Marketing decides that they NEED an MTBF number to satisfy the customers,
usually because the compettitor has one. They don't want to pay the $$$ to
really do the job (and they have no idea of the size of the job!), but they
want you to "tell them" what the MTBF is.

GAAARRRRGH.

I agree with Nigel, if your defects are scattered, and there aren't a lot of
them, then you're in fine shape. If you see consistent failures in one area
or component, then you need to investigate why.

When I roll out a new product, I insist on seeing all the failed units until
I'm satisfied that they are right.  The repair guys just fix, and they
aren't going to be looking for what caused the failure, that is an
engineer's job.