• Mil-HDBK-217F: Reliability Prediction of Electronic Equipment, which was developed by the United States Department of Defense (DOD) in the 1960s. This standard is used in military and commercial applications throughout the world and covers 14 separate operational environments, it has not been updated since 1995.
• Bellcore Standard: The Bellcore approach is widely used in the telecommunications industry, and covers five separate use environments. It is very similar to MIL-HDBK-217. 

For example, if you want to calculate the MTBF of IC, MIL-HDBK-217 approach , you need to know parameters like: Package, pin number, power max dissipated, thermal resistance, etc. The same chip in Bellcore approach, you need to know less parameters like: package, IC Technologies, transistor or gate or bit number. For the Bellcore standard you need less parameters to calculate the MTBF parameter. For more detailed information, in the section Reliability prediction, sub-section Environmental conditions there is a brief description of environmental conditions for Bellcore and MIL-S217. Regarding MTTR, this parameter is defined as: the average time it takes to repair a failed component. The time to repair include several activities like: Preparation time, active maintenance time and delay time (logistics time). There are 4 methods to predict the MTTR of a system, but the most used is based on summing the product of the expected repair times of the individual failures modes (Full Maintenance Time - FMT) and dividing by the sum of the individual failure rates. When you perform MTTR calcultion you build a top down tree with block and sublock. If a block has not sub blocks (leaf block), this means it is replaced as a whole unit when fails and is discarded. Its Full Maintenance Time (FMT) includes only replacement operations named Maintenance Time (MT): disconnection, detection, replacement, connection and testing. If a block has sub blocks, then it is repaired, e.g. fail sub blocks are removed from it and repaired or replaced. There may be 2 cases: 

• The block is replaced before repair by a spare part. Its FMT includes only replacement operations time (MT). Its repair may be performed separately at the same or at other Maintenance level while the system continues operation. 
• The block is not replaceable. The system waits for the block repair completion to continue operation. The block FMT includes its own MT (disconnection, detection, connection and testing operations time, similarly to replaced block) and, in addition, the Mean Time To failed sub blocks Repair or Replacement (MTTR). So, the block MTTR in this program is defined as average block repair time as separate sub system at its repair site. It does not include removing from the system, connection to the system after repair, testing, etc .