Software reliability engineering relies heavily on a disciplined software engineering process to anticipate and design against unintended consequences. The origins of contemporary reliability engineering can be traced to World War II. Any type of reliability requirement should be detailed and could be derived from failure analysis (Finite-Element Stress and Fatigue analysis, Reliability Hazard Analysis, FTA, FMEA, Human Factor Analysis, Functional Hazard Analysis, etc.) So, the reliability of a software product is more about the random discovery of faults resulting from various inputs with the system in various states. However, even if no individual part of the system fails, but the system as a whole does not do what was intended, then it is still charged against the system reliability. Repairing or replacing the hardware component restores the system to its original operating state. System reliability, by definition, includes all parts of the system, including hardware, software, supporting infrastructure (including critical external interfaces), operators and procedures. These should be written by trained or experienced technical authors using so-called simplified English or Simplified Technical English, where words and structure are specifically chosen and created so as to reduce ambiguity or risk of confusion (e.g. A reliability program plan may also be used to evaluate and improve the availability of a system by the strategy of focusing on increasing testability & maintainability and not on reliability. Reliability and availability models use block diagrams and Fault Tree Analysis to provide a graphical means of evaluating the relationships between different parts of the system. The non-functional requirements are also called Quality attributes of the software under development. Musa, John (2005) Software Reliability Engineering: More Reliable Software Faster and Cheaper, 2nd. Reliability testing is common in the Photonics industry. Furthermore, reliability design requirements should drive a (system or part) design to incorporate features that prevent failures from occurring, or limit consequences from failure in the first place. Mean Time Between Failure (MTBF)=Mean Time To Failure (MTTF)+ Mean Time To Repair (MTTR) A single test is in most cases insufficient to generate enough statistical data. However, software does not fail in the same sense that hardware fails. Establish quality and reliability requirements for suppliers. Reliability applies to a specified period of time. In such cases, different approaches to testing can be used, such as (highly) accelerated life testing, design of experiments, and simulations. Six Sigma may also help to design products that are more robust to manufacturing induced failures. Comparing different types of causes may lead to incorrect estimations and incorrect business decisions about the focus of improvement. Reliability may be defined in the following ways: Many engineering techniques are used in reliability risk assessments, such as reliability hazard analysis, failure mode and effects analysis (FMEA), Consistent with the creation of a safety cases, for example ARP4761, the goal of reliability assessments is to provide a robust set of qualitative and quantitative evidence that use of a component or system will not be associated with unacceptable risk. This activity reviews all requirements and may provide a graphical view of the entire system. One strategy to address this issue is to use a scoring conference process. In software engineering, dependability is the ability to provide services that can defensibly be trusted within a time-period. There is risk of incorrectly accepting a bad design (type 1 error) and the risk of incorrectly rejecting a good design (type 2 error). As an example, the failure of the tail-light of an aircraft will not prevent the plane from flying (and so is not considered a mission failure), but it does need to be remedied (with a related cost, and so does contribute to the basic unreliability levels). For the users of a system it is the reliability of the system as a whole that is meaningful but for analysts and testers it is important to separate the software requirements from the hardware requirements as there are some significant differences. 12. When reliability is not under control, more complicated issues may arise, like manpower (maintainers / customer service capability) shortages, spare part availability, logistic delays, lack of repair facilities, extensive retro-fit and complex configuration management costs, and others. via different suppliers of similar parts) for single independent channels, can provide less sensitivity to quality issues (e.g. I agree to Seilevel’s Privacy Policy *. failure rates for a particular failure mode or event and the mean time to repair the system for a particular failure). Furthermore, the most unreliable and important items (i.e. Determine the best mitigation and get agreement on final, acceptable risk levels, possibly based on cost/benefit analysis. The development team may refine the environmental and use conditions specific for the element, in this case, a circuit board in order to fully describe the reliability requirements. While the (input data) predictions are often not accurate in an absolute sense, they are valuable to assess relative differences in design alternatives. and Marais, Ken, 'Highlights from the Early (and pre-) History of Reliability Engineering', Reliability Engineering and System Safety, Volume 91, Issue 2, February 2006, Pages 249–256, Juran, Joseph and Gryna, Frank, Quality Control Handbook, Fourth Edition, McGraw-Hill, New York, 1988, p.24.3, Wong, Kam, 'Unified Field (Failure) Theory-Demise of the Bathtub Curve', Proceedings of Annual RAMS, 1981, pp402-408, Practical Reliability Engineering, P. O'Conner – 2012, Using Failure Modes, Mechanisms, and Effects Analysis in Medical Device Adverse Event Investigations, S. Cheng, D. Das, and M. Pecht, ICBO: International Conference on Biomedical Ontology, Buffalo, NY, July 26–30, 2011, pp. Furthermore, human errors in management; the organization of data and information; or the misuse or abuse of items, may also contribute to unreliability. The input for the models can come from many sources including testing; prior operational experience; field data; as well as data handbooks from similar or related industries. These are devices or systems that remain relatively dormant and only operate once. More inherent aspects may play a role, and in some cases, these may not be readily measured or controlled by any means. The product either supports the requirement or it does not and therefore either passes or fails the test. The term was first used by IBM to define specifications for their mainframe s and originally applied only to hardware . The basic steps to take. The core of Six-Sigma is built on empirical research and statistical analysis (e.g. Many of the tasks, techniques, and analyses used in Reliability Engineering are specific to particular industries and applications, but can commonly include: Results from these methods are presented during reviews of part or system design, and logistics. We’ve already covered different types of software requirements, but this time we’ll focus on non-functional ones, and how to approach and document them. The complexity of the technical systems such as improvements of design and materials, planned inspections, fool-proof design, and backup redundancy decreases risk and increases the cost. fractured item) in reliability. Six-Sigma is also more quantified (measurement-based). This is common practice in Aerospace systems that need continued availability and do not have a fail-safe mode. The reliability program also includes a systematic root cause analysis that identifies the causal relationships involved in the failure such that effective corrective actions may be implemented. Statistical confidence is increased by increasing either the test time or the number of items tested. Todinov, M. (2016), 'Reliability and Risk Models: setting reliability requirements', Wiley, 978-1-118-87332-8. These requirements are generally specified in the contract statement of work and depend on how much leeway the customer wishes to provide to the contractor. Organizations today are adopting this method and utilizing commercial systems (such as Web-based FRACAS applications) that enable them to create a failure/incident data repository from which statistics can be derived to view accurate and genuine reliability, safety, and quality metrics. This can not be translated practically to most reliability issues, as reliability is not (easily) measurable due to being very much a function of time (large times may be involved), especially during the requirements-specification and design phases, where reliability engineering is the most efficient. System availability and mission readiness analysis and related reliability and maintenance requirement allocation, Functional system failure analysis and derived requirements specification, Inherent (system) design reliability analysis and derived requirements specification for both hardware and software design, Fault tolerant systems (e.g. High reliability (safety factor) levels also result from good engineering and from attention to detail, and almost never from only reactive failure management (using reliability accounting and statistics). That same rover may be required to operate in varying conditions requiring additional scrutiny. Other software metrics, such as complexity, are also used. Full quantification of reliability is in this phase extremely difficult or costly (due to the amount of testing required). Software Requirements Specifications, also known as SRS, is the term used to describe an in-depth description of a software product to be developed. requirements, design changes, detection logic, maintenance, training, by which the risks may be lowered and controlled for at an acceptable level. That is where 'reliability' comes in. ISBN. For electronic assemblies, there has been an increasing shift towards a different approach called physics of failure. In other cases, reliability is specified as the probability of mission success. RAMT stands for reliability, availability, maintainability/maintenance, and testability in the context of the customer's needs. A diverse set of practical guidance as to performance and reliability should be provided to designers so that they can generate low-stressed designs and products that protect, or are protected against, damage and excessive wear. Reliability requirements are included in the appropriate system or subsystem requirements specifications, test plans, and contract statements. Reliability engineering is used to design a realistic and affordable test program that provides empirical evidence that the system meets its reliability requirements. Media related to Reliability engineering at Wikimedia Commons. Different test plans result in different levels of risk to the producer and consumer. Maintainability parameters, for example Mean time to repair (MTTR), can also be used as inputs for such models. Within an entity, departments related to Quality (i.e. Reliability engineering is a specialty engineering part of systems engineering. There is more overlap between software quality engineering and software reliability engineering than between hardware quality and reliability. Some of the most common methods to apply to a reliability operational assessment are failure reporting, analysis, and corrective action systems (FRACAS). The full mathematical quantification (in statistical models) of this combined relation is in general very difficult or even practically impossible. If you’ve ever dealt with non-functional requirements, you may know that differen… Tests for functional requirements are usually binary. The risk can be decreased to ALARA (as low as reasonably achievable) or ALAPA (as low as practically achievable) levels. The scoring conference process is defined in the statement of work. early childhood failures at a single supplier), allowing very-high levels of reliability to be achieved at all moments of the development cycle (from early life to long-term). Although this may seem obvious, there are many situations where it is not clear whether a failure is really the fault of the system. Setting only availability, reliability, testability, or maintainability targets (e.g., max. Bagdonavicius, V., Nikulin, M., (2002), 'Accelerated Life Models. Evidence can be generated with some level of confidence by testing. incorrect load settings or failure measurement), Feedback of field information (e.g. The desired reliability, statistical confidence, and risk levels for each side influence the ultimate test plan. 2oo3 voting logic) where multiple redundant elements must agree on a potentially unsage action before it is performed. At a component level, the same types of analyses can be used together with others. Items can, however, fail over time, even if these requirements are all fulfilled. But, as GM and Toyota have belatedly discovered, TCO also includes the downstream liability costs when reliability calculations have not sufficiently or accurately addressed customers' personal bodily risks. To perform a proper quantitative reliability prediction for systems may be difficult and very expensive if done by testing. For systems in dormant storage or on standby, it is necessary to establish a formal surveillance program to inspect and test random samples. Will these result in premature failure due to excessive wear, fatigue, corrosion, and debris accumulation, or other issues such as maintenance induced failures? to find transfer functions) of directly measurable parameters. Six Sigma has its roots in manufacturing. Software Engineering / Software Reliability / 11. The minimum hardware/software requirements for each server hosting the database should be obtained directly from the selected database vendor (i.e., Microsoft or Oracle). Testability (not to be confused with test requirements) requirements provide the link between reliability and maintainability and should address detectability of failure modes (on a particular system level), isolation levels, and the creation of diagnostics (procedures).As indicated above, reliability engineers should also address requirements for various reliability tasks and documentation during system development, testing, production, and operation. In 1945, M.A. Following the incorrect route of trying to quantify and solve a complex reliability engineering problem in terms of MTBF or probability using an-incorrect – for example, the re-active – approach is referred to by Barnard as 'Playing the Numbers Game' and is regarded as bad practice. Some tasks are better performed by humans and some are better performed by machines. Not only would it aid in some predictions, this effort would keep from distracting the engineering effort into a kind of accounting work. The maintenance strategy can influence the reliability of a system (e.g., by preventive and/or predictive maintenance), although it can never bring it above the inherent reliability. The MTTF is usually specified in hours, but can also be used with other units of measurement, such as miles or cycles.
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