Definition 1.1 says that reliability expresses “the ability of an item to perform as required in a stated operating context and for a stated period of time.” We start by clarifying the main words in this definition.
1 Reliability is defined by using the word ability, which is not directly measurable. A quantitative evaluation of the item's ability to perform must therefore be based on one or more metrics, called reliability metrics. Several probabilistic reliability metrics are defined and discussed in Section 1.4.
2 Some authors use the word capability instead of ability in the definition of reliability and claim that the term “capability” is more embracing, covering both ability and capacity. Most dictionaries list ability and capability as synonyms. We prefer the word “ability” because this is the word most commonly used.
3 The statement perform as required means that the item must be able to perform one or more specified functions according to the performance criteria for these function(s). Functions and performance criteria are discussed in Section 2.5.
4 Many items can perform a high number of functions. To assess the reliability (e.g. of a car), we must specify the required function(s) that are considered.
5 To be reliable, the item must do more than meet an initial factory performance or quality specification – it must operate satisfactorily for a specified period of time in the actual operating context.
6 The stated period of time may be a delimited time period, such as a mission time, the time of ownership, and several more.
7 The time may be measured by many different time concepts, such as calendar time, time in operation, number of work cycles, and so on. For vehicles, the time is often measured as the number of kilometers driven. For items that are not operated continuously in the same mode, a more complicated time concept may be needed.
Inherent and Actual Reliability
It may be useful to qualify the reliability of an item by adding a word, such as inherent or actual. The inherent reliability is defined as follows:
Definition 1.3 (Inherent reliability)
The reliability of the item as designed and manufactured, which excludes effects of operation, environment, and support conditions other than those assumed and stated in the item requirements and specification.
The inherent reliability is therefore the reliability of a brand new item that will be used and maintained exactly according to the conditions described in the item specification document or implicitly assumed. The inherent reliability is sometimes called built reliability or built‐in reliability of the item.
The design and development team always attempts to adapt the item to the actual operating context, but it is difficult, if not impossible, to account for all the aspects in practical use. The actual reliability may consequently be different from the inherent reliability that was determined before the item was put into use. The actual reliability of an item is defined as follows:
Definition 1.4 (Actual reliability)
The reliability of the item in an actual operating context.
The actual reliability is sometimes called operational reliability or functional reliability.
Software Reliability
Software reliability is different from hardware reliability. Hardware items generally deteriorate due to wear or other mechanisms and failures occur as a random process. Software, on the other hand, does not deteriorate and faults or bugs remain dormant and undetected until the software is modified or a specific condition or trigger activates the bug – leading to item failure. Software bugs are manifestations of mistakes done in specification, design, and/or implementation. Reliability analysis of a software program is done by checking the code syntax according to specific rules and by testing (debugging) the software for a variety of input data. This process is not discussed further in this book. Interested readers may consult ISO 25010.
1.3.2 Maintainability and Maintenance
Many items have to be maintained to perform as required. Two different concepts are important, maintainability, and maintenance. Maintainability is a design feature of the item and indicates how easy it is to get access to the parts that are to be maintained and how fast a specific maintenance task can be done. Maintenance describes the actual work that is done to maintain an item. Maintainability is defined as follows:
Definition 1.5 (Maintainability)
The ability of an item, under stated conditions of use, to be retained in, or restored to, a state in which it can perform as required, when maintenance is performed under stated conditions and using prescribed procedures and resources.
Maintainability is further discussed in Chapter 9. Maintenance is defined as follows:
Definition 1.6 (Maintenance)
The combination of all technical and management actions during the life cycle of an item intended to retain the item in, or restore it to, a state in which it can perform as required (IEV 192‐06‐01).
Hardware maintenance is discussed in more detail in Chapters 9 and 12. Software maintenance is not treated in this book.
1.3.3 Availability
Availability measures the degree to which an item is able to operate at some future time
1.3.4 Quality
The term “quality” is closely related to reliability and is defined as follows:
Definition 1.7 (Quality)
The totality of features and characteristics of a product or service that bear on its ability to satisfy stated or implied needs.
Quality is sometimes defined as conformity to specifications and a quality defect is referred to as a nonconformity. According to common usage, quality denotes the conformity of the item to its specification as manufactured, whereas reliability denotes its ability to continue to comply with its specification over its
