This International Standard provides mathematical expressions for reliability, availability, maintainability and maintenance support measures defined in IEC 60050-191. The following classes of items are considered separately in this standard: - non-repaired items; - repaired items with zero time to restoration; - repaired items with non-zero time to restoration. In order to keep the mathematical formulae as simple as possible, the following basic mathematical models are used to quantify dependability measures: - random variable (time to failure) for non-repaired item; - simple (ordinary) renewal process for repaired item with zero time to restoration; - simple (ordinary) alternating renewal process for repaired items with non-zero time to restoration. To facilitate location of the full definition, the IEC 60050-191 reference for each term is shown (in parenthesis) immediately following each term, for example: mean time to restoration [191-13-08] The application of each dependability measure is illustrated by means of simple example. NOTE this standard is mainly applicable to hardware dependability, but many terms and their definitions may be applied to items containing software. Some of the sotware dependability aspects are explained in annex D.