This is the first key question which leads to understanding reliability. At TriQuint, failures are considered to be an essential part of each reliability study. Failures are required to identify root causes of reliability problems, or to evaluate the weakest link of an IC so that improvements can be leveraged for the best impact on reliability.
Failures also provide the reference point for future comparisons.
If subsequent tests are run before and after a process change, and they both result in zero failures, there's no way to decide if the process change improved or impaired the reliability. It's auspicious that the TriQuint data base contains over 4,000 failures from the various tests that have been completed. Just any old failure won't do, however. Sinking gates are caused by gate metal interdiffusion into the channel. The largest change is decreased channel current so that parameter is typically used as the failure criteria.