DSIAC was asked to provide some assessment on the general acceptance of using MTBUR and MTBD instead of MTBF, using “defect” instead of “failures.” DSIAC reached out to a reliability subject matter expert, who gave the following response.
Regarding use of MTBUR – first, in terms of failures, MTBUR only addresses failures that can be repaired by replacing the failed item. Some failures require a repair in place, not a removal. Second, MTBUR can refer to the removal of items that have not failed but must be removed to access a failed item.
Regarding defect vs. failure – first, a defect can be, but is not necessarily, a failure. For example, an imperfection in a finish may not look good but may not affect the function of the item. Failure means that an item cannot perform at least one of its functions. Second, a defect is more closely associated with quality than it is with reliability. Failure, on the other hand, is a term used in the very definition of reliability (reliability is the ability of an item to perform its function(s) without failure under stated conditions for a specified period of time).