Random Error Waiting Time
Part of the way I did DVT's different from everyone else was to verify the all the design specs including HALT/HASS with margin measurements, not just pass-fail.  

The objective is not unique to raise stress levels to a non-destructive fault level but rather find a way to find how to measure the margin to error or functional failure. 

This can be accomplished by injecting radiated and conducted &amp; radiated noise of all types; RF, ESD, arc noise, thermal and vibration, and injecting false data on the interface etc.

If you can measure margin to failure at all the Environmental stress levels expected (Climatic, mechanical, electrical, human error, etc( then find the weakest links). 

Hopefully, you have some way to automate the measurements with built-in self-test.

Proving that a new product means verify the design, process and components are all defects free with adequate margin. There are statistical ways to measure this before acceptance such as Dpk based on std dev and min-max, fault detection and correction coverage, etc.

Your criteria for acceptance ( parameters, methods and values)  must be defined by detailed design specs in order to have a reliable product.

My favorite final process test for new SMD MOBO'S was random vibration with heat on a fixture or a vibrator on backplane cards.   But for high-speed synchronous links prone to errors such as magnetic disk drives, I used SERDES "window margin", a digital form of eye pattern on worst-case data patterns.

For each project a DVT might be 30 pages with 1 page per test such as climatic. I spent 8 years as Test Eng. Mgr responsible for conducting DVT's on new products and all factory production with over 20 mainframes testing diskdrives and 100's of special test gear with a dozen ATE for board test, and Burn-in heated chambers for every system final test in production at 40'C