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is metric along with software execution time is key to most software reliability models and estimates the theory is that the software reliability increases as the number of faults or fault density decreases establishing a direct connection between fault density and mean time between failure is difficult however because of the way software faults are distributed in the code their severity and the probability of the combination of inputs necessary to encounter the fault nevertheless fault density serves as a useful indicator for the reliability engineer other software metrics such as complexity are also used this metric remains controversial since changes in software development and verification practices can have dramatic impact on overall defect rates software testing is an important aspect of software reliability even the best software development process results in some software faults that are nearly undetectable until tested software is tested at several levels starting with individual units through integration and full up system testing all phases of testing software faults are discovered corrected and re tested reliability estimates are updated based on the fault density and other metrics at a system level mean time between failure data can be collected and used to estimate reliability unlike hardware performing exactly the same test on exactly the same software configuration does not provide increased statistical confidence instead software reliability uses different metrics such as code coverage the software engineering institute s capability maturity model is a common means of assessing the overall software development process for reliability and quality purposes structural reliability edit main article structural reliability structural reliability or the reliability of structures is the application of reliability theory to the behavior of structures it is used in both the design and maintenance of different types of structures including concrete and steel structures 38 39 in structural reliability studies both loads and resistances are modeled as probabilistic variables using this approach the probability of failure of a structure is calculated comparison to safety engineering edit main article safety engineering reliability for safety and reliability for availability are often closely related lost availability of an engineering system can cost money if a subway system is unavailable the subway operator will lose money for each hour the system is down the subway operator will lose more money if safety is compromised the definition of reliability is tied to a probability of not encountering a failure a failure can cause loss of safety loss of availability or both it is undesirable to lose safety or availability in a critical system reliability engineering is concerned with overall minimisation of failures that could lead to financial losses for the responsible entity whereas safety engineering focuses on minimising a specific set of failure types that in general could lead to loss of life injury or damage to equipment reliability hazards could transform into incidents leading to a loss of revenue for the company or the customer for example due to direct and indirect costs associated with loss of production due to system unavailability unexpected high or low demands for spares repair costs man hours re designs or interruptions to normal production 40 safety engineering is often highly specific relating only to certain tightly regulated industries applications or areas it primarily focuses on system safety hazards that could lead to severe accidents including loss of life destruction of equipment or environmental damage as such the related system functional reliability requirements are often extremely high although it deals with unwanted failures in the same sense as reliability engineering it however has less of a focus on direct costs and is not concerned with post failure repair actions another difference is the level of impact of failures on society leading to a tendency for strict control by governments or regulatory bodies e g nuclear aerospace defense rail and oil industries 40 fault tolerance edit main article fault tolerance safety can be increased using a 2oo2 cross checked redundant system availability can be increased by using 1oo2 1 out of 2 redundancy at a part or system level if both redundant elements disagree the more permissive element will maximize availability a 1oo2 system should never be relied on for safety fault tolerant systems often rely on additional redundancy e g 2oo3 voting logic where multiple redundant elements must agree on a potentially unsafe action before it is performed this increases both availability and safety at a system level this is common practice in aerospace systems that need continued availability and do not have a fail safe mode for example aircraft may use triple modular redundancy for flight computers and control surfaces including occasionally different modes of operation e g electrical mechanical hydraulic as these need to always be operational due to the fact that there are no safe default positions for control surfaces such as rudders or ailerons when the aircraft is flying basic reliability and mission reliability edit the above example of a 2oo3 fault tolerant system increases both mission reliability as well as safety however the basic reliability of the system will in this case still be lower than a non redundant 1oo1 or 2oo2 system basic reliability engineering covers all failures including those that might not result in system failure but do result in additional cost due to maintenance repair actions logistics spare parts etc for example replacement or repair of 1 faulty channel in a 2oo3 voting system the system is still operating although with one failed channel it has actually become a 2oo2 system is contributing to basic unreliability but not mission unreliability 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 detectability and common cause failures edit when using fault tolerant redundant systems or systems that are equipped with protection functions detectability of failures and avoidance of common cause failures becomes paramount for safe functioning and or mission reliability reliability versus quality six sigma edit main article six sigma quality often focuses on manufacturing defects during the warranty phase reliability looks at the failure intensity over the whole life of a product or engineering system from commissioning to decommissioning six sigma has its roots in statistical control in quality of manufacturing reliability engineering is a specialty part of systems engineering the systems engineering process is a discovery process that is often unlike a manufacturing process a manufacturing process is often focused on repetitive activities that achieve high quality outputs with minimum cost and time 41 the everyday usage term quality of a product is loosely taken to mean its inherent degree of excellence in industry a more precise definition of quality as conformance to requirements or specifications at the start of use is used assuming the final product specification adequately captures the original requirements and customer system needs the quality level can be measured as the fraction of product units shipped that meet specifications 42 manufactured goods quality often focuses on the number of warranty claims during the warranty period quality is a snapshot at the start of life through the warranty period and is related to the control of lower level product specifications this includes time zero defects i e where manufacturing mistakes escaped final quality control in theory the quality level might be described by a single fraction of defective products reliability as a part of systems engineering acts as more of an ongoing assessment of failure rates over many years theoretically all items will fail over an infinite period of time 43 defects that appear over time are referred to as reliability fallout to describe reliability fallout a probability model that describes the fraction fallout over time is needed this is known as the life distribution model 42 some of these reliability issues may be due to inherent design issues which may exist even though the product conforms to specifications even items that are produced perfectly will fail over time due to one or more failure mechanisms e g due to human error or mechanical electrical and chemical factors these reliability issues can also be influenced by acceptable levels of variation during initial production quality and reliability are therefore related to manufacturing reliability is more targeted towards clients who are focused on failures throughout the whole life of the product such as the military airlines or railroads items that do not conform to product specification will generally do worse in terms of reliability having a lower mttf but this does not always have to be the case the full mathematical quantification in statistical models of this combined relation is in general very difficult or even practically impossible in cases where manufacturing variances can be effectively reduced six sigma tools have been shown to be useful to find optimal process solutions which can increase quality and reliability six sigma may also help to design products that are more robust to manufacturing induced failures and infant mortality defects in engineering systems and manufactured product in contrast with six sigma reliability engineering solutions are generally found by focusing on reliability testing and system design solutions are found in different ways such as by simplifying a system to allow more of the mechanisms of failure involved to be understood performing detailed calculations of material stress levels allowing suitable safety factors to be determined finding possible abnormal system load conditions and using this to increase robustness of a design to manufacturing variance related failure mechanisms furthermore reliability engineering uses system level solutions like designing redundant and fault tolerant systems for situations with high availability needs see reliability engineering vs safety engineering above note a defect in six sigma quality literature is not the same as a failure field failure e g fractured item in reliability a six sigma quality defect refers generally to non conformance with a requirement e g basic functionality or a key dimension items can however fail over time even if these requirements are all fulfilled quality is generally not concerned with asking the crucial question are the requirements actually correct whereas reliability is reliability operational assessment edit once systems or parts are being produced reliability engineering attempts to monitor assess and correct deficiencies monitoring includes electronic and visual surveillance of critical parameters identified during the fault tree analysis design stage data collection is highly dependent on the nature of the system most large organizations have quality control groups that collect failure data on vehicles equipment and machinery consumer product failures are often tracked by the number of returns for systems in dormant storage or on standby it is necessary to establish a formal surveillance program to inspect and test random samples any changes to the system such as field upgrades or recall repairs require additional reliability testing to ensure the reliability of the modification since it is not possible to anticipate all the failure modes of a given system especially ones with a human element failures will occur 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 when possible system failures and corrective actions are reported to the reliability engineering organization some of the most common methods to apply to a reliability operational assessment are failure reporting analysis and corrective action systems fracas this systematic approach develops a reliability safety and logistics assessment based on failure incident reporting management analysis and corrective preventive actions 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 it is extremely important for an organization to adopt a common fracas system for all end items also it should allow test results to be captured in a practical way failure to adopt one easy to use in terms of ease of data entry for field engineers and repair shop engineers and easy to maintain integrated system is likely to result in a failure of the fracas program itself some of the common outputs from a fracas system include field mtbf mttr spares consumption reliability growth failure incidents distribution by type location part no serial no and symptom the use of past data to predict the reliability of new comparable systems items can be misleading as reliability is a function of the context of use and can be affected by small changes in design manufacturing reliability organizations edit systems of any significant complexity are developed by organizations of people such as a commercial company or a government agency the reliability engineering organization must be consistent with the company s organizational structure for small non critical systems reliability engineering may be informal as complexity grows the need arises for a formal reliability function because reliability is important to the customer the customer may even specify certain aspects of the reliability organization there are several common types of reliability organizations the project manager or chief engineer may employ one or more reliability engineers directly in larger organizations there is usually a product assurance or specialty engineering organization which may include reliability maintainability quality safety human factors logistics etc in such case the reliability engineer reports to the product assurance manager or specialty engineering manager in some cases a company may wish to establish an independent reliability organization this is desirable to ensure that the system reliability which is often expensive and time consuming is not unduly slighted due to budget and schedule pressures in such cases the reliability engineer works for the project day to day but is actually employed and paid by a separate organization within the company because reliability engineering is critical to early system design...
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