MTBF

I don't know whether there is a newer method, but one method I used is Military Handbook 217D (MIL-HDBK-217D). It assigns component failure rates for hundreds of different types of parts (resistors, transformers, IC's, etc.) and then you have add the faiure rates and MTBF was caculated by 1 million divided by failure rate sum (comes out in hours). In a nutshell, MTBF becomes lower when 1) more components are used, 2)if fragile components (with higher failure rate number) are used, 3) when operated at abnormal temperature (about 10 degree C ambient temperature is where maximum MTBF is normally achieved).

If there is a newer MTBF calculation method, I would like to know about it, too.