In that they employ similar methodologies for analysis and may require input from one another, reliability engineering is closely related to quality engineering, safety engineering, and system safety. It is possible to state that a system must be dependably safe. Dependability is the ability of something to perform its function under specified conditions for which it was not designed. Thus, reliability and quality are two different but related aspects of system design.
Reliability and quality both focus on how well a product or service meets its intended purpose. However, while quality also takes into account user experience (e.g., ease of use, aesthetics), reliability focuses solely on the functional integrity of the product over its lifetime. For example, a quality product might have some issues upon arrival that need to be resolved before it can be used reliably, while a reliable product could have problems that prevent it from being considered quality.
Quality and reliability each have several specific meanings in business. As general terms, they describe a product or service that performs as expected during normal use. However, if problems do arise, they should be able to be fixed easily with little impact on the overall quality or reliability of the product. A high-quality product is one that satisfies users needs and expectations without any major defects. A high-reliable product does what it is supposed to do most of the time but may occasionally have problems due to manufacturing errors or aging components.
Reliability engineering is a branch of systems engineering that focuses on the capacity of equipment to operate without failure. The capacity of a system or component to work under defined conditions for a certain amount of time is referred to as reliability. Reliability is different from performance. A high-performance component may fail after a few hours of use. A reliable component will usually function properly for many years.
In general, the higher the quality rating of a product, the more expensive it is, which means that less expensive products tend to be lower quality than higher price ones. For example, a television set can be rated either "CE" (for consumer electronics) or "RLC" (for radio-frequency communication). These ratings tell us how well the product meets government safety standards and performs other functions it must meet before being sold to the public. Quality control tests are also used to ensure that products perform according to their specifications for a certain period of time. For example, cars are tested to make sure that they will start every time you turn the key in the ignition, that the tires don't go flat even though they appear fine, and so on.
Engineers design products with the assumption that some parts will fail during this time. They try to build these parts so that they have enough strength to last for several years of use.
Engineering for dependability. Dependability, often known as reliability, refers to a system's or component's ability to work under given conditions for a set amount of time. Reliability is closely connected to availability, which is often defined as a component's or system's capacity to work at a specific time or interval of time. Engineering dependable systems requires balancing tradeoffs between design options that might increase reliability (e.g., using more robust components) and performance (e.g., speed). Designing for low reliability may require making some features optional or providing several designs for different levels of reliability.
In general, a reliable system does not fail most of the time. A system fails when it cannot complete its task within an acceptable period of time. Availability and reliability are two very different things. An available system can fail if something goes wrong while it is being used; a reliable system will always perform its function even if something goes wrong during periods when it is not being used.
Reliability theory helps us understand what factors affect the reliability of a system. According to this theory, reliability depends on three main factors: complexity, defects, and maintenance. Complex systems are hard to make reliable because there are many ways in which they can malfunction. Simple systems tend to suffer only from the obvious failure modes of their components. Defects are deviations from perfect condition, such as cracks in mechanical parts or open circuits in electrical components. Defects can cause systems to malfunction prematurely or violate safety constraints.