How to estimate and use MTTF/MTBF would the real MTBF please stand up?

This paper discusses, with examples, uses of the terms mean time to failure (MTTF) and mean time between failures (MTBF) in a variety of contexts. Especially, this paper points out how the same terms are explained and understood in a variety of meanings, and in very many cases are misinterpreted and misunderstood. Both terms, MTBF and MTTF are used as an attribute or measure of hardware and software reliability, and also as essential information in logistics for planning of the spare parts for the repairable items, or planning of the replacement or non repairable items, warranties, planning for part obsolescence, and such. It is a very frequent practice that MTTF is understood to be the universal attribute of a non-repairable item, and in the gross simplification it is often assumed to be the indicator of the expected life of that item. In a multitude of occasions, it is used interchangeably with the MTBF, and with no mention of how this value was derived, for what circumstances, for what stresses, or its application. The values are advertized, and the goodness of the product is judged by the advertized values. This paper does not focus on the correct use of both measures (MTTF and MTTF). It merely discusses the often inaccurate applications and often gross misinterpretation of those terms. In particular, this paper discusses their actual meaning, the dependency of their values on use of the items within products or by themselves, the meaning or the lack of it for various products of current technology, and the misguide they can provide if used for maintenance or repair planning. The paper emphasizes that one item does not have and cannot have one MTBF or MTTF, but those, if calculated, averaged, estimated, are entirely and widely dependent on the application stresses, their use profile within a system, the period of their use, the construction of the products, their sensitivity to the stress changes etc. Only physically correct mathematical representation may be able to decide usefulness and reliability of items, dependent on their application and the needs of the products where they are used. Also, the MTBF is a measure of failure frequency thus is dependent for the major part on the MTTF of the least reliable part in that item. The paper also explains the meaning of the reliability test results related to the MTBF or MTTF of an item and to the reliability of its components.