2 Outline Six Sigma Software Reliability Failure Measures of Reliability & AvailabilitySoftware SafetyQuality StandardsSQA Plan
3 Six Sigma …It is the most widely used strategy used in industry for statistical quality assurance.It was originally popularized by Motorola in 1980s.It can be described asA rigorous and disciplined methodology that uses data and the statistical analysis to measure & improve a company’s operational performance by identifying and eliminating ‘defects’ in manufacturing & service-related processes
4 Six Sigma (Contd.) Six sigma methodology defines 3 core steps: Define customer requirements, deliverables, & project goals via well defined methods of customer comm.Measure the existing process & its output to determine current quality performance (collect defect metrics)Analyze defect metrics & determine the vital few causes.
5 Six Sigma (Contd.)If an existing software process is in place, but improvement is required, Six Sigma suggests 2 additional steps:Improve the process by eliminating the root causes of defectsControl the process to ensure that future work does not reintroduce the causes of defectsThese core steps and additional steps are also referred to as DMAIC method
6 Six Sigma (Contd.)If an organization is developing a software process (rather than improving an existing one), the core steps are augmented by:Design the process to (1) avoid the root causes of defects and (2) to meet the customer requirementsVerify that the process model will, in fact, avoid defects and meet customer requirementsThis is referred to as DMADV method
7 Software Reliability Definition The probability of failure free operation of a computer program in a specified environment for a specified time.Reliability is the probability of not failing in a specified length of time
8 Software Reliability (Contd.) Mathematical representationF(n) = 1 - R(n)Where,R(n) = probability of reliability (i.e. not failing)n = no. of time units,if time unit is assumed in days then probability of not failing in 1 day is R(1)F(n) = probability of failing in a specified length of time
9 Software Reliability (Contd.) It is a quality factor that can be directly measured and estimated using historical ‘development data’.It measures how often s/w encounters a data input or other condition that it does not correctly process to produce correct answerIf programX has reliability of 0.96 (over 8 processing hours) then it means, if programX runs 100 times – it will operate correctly 96 times
10 Failure Non-conformance to s/w requirements leads to failures Negative results or in worst case no output is failureSome failures can be corrected in seconds, some in weeks and others in monthsOne failure may introduce other errors (in effect other failures)
11 Measures of Reliability & Availability Early work in software reliability attempted to extrapolate the mathematics of hardware reliability theory to prediction of software reliability. But,Most hardware reliability models have predicted on failure occur due to physical wear (corrosion effects, shock, temperature, etc.) rather than design defects.The opposite is true for softwares. All software failures can be traced to design or implementation problems.
12 Measures of Reliability & Availability Measure of ReliabilityConsider a computer-based system.A simple measure of reliability for such a system is mean-time-between-failure (MTBF)MTBF = MTTF + MTTRMTTF = Mean-time-to-failureMTTR = Mean-time-to-repairMany researchers argue that MTBF is more useful term than defects/KLOC or defects/FP as user is more concerned with failure rate as compared to defect count.Each defect does not have same failure rate and the total defect count gives little indication of the reliability of a system
13 Measures of Reliability & Availability Measure of AvailabilitySoftware availability is the probability that a program is operating according to requirements at a given point in time.It is defined asAvailability = [MTTF / (MTTF + MTTR] * 100%Availability measure is sensitive to MTTR
14 Software SafetyThis SQA activity focuses on identification & assessment of potential hazards that may affect software negatively & cause an entire system to fail.Early identification of hazards can help to have design features that either eliminate or control potential hazards.A modeling & analysis process is conducted as part of s/w safety.Initially – hazards identified & categorized by criticality & riskNext – analysis techniques are used to assign severity & probability of occurrence (similar to risk analysis methods but different as the emphasis in this case is on technology issues rather than project )
15 Software Safety (Contd.) The following analysis techniques can be used:Fault tree analysisReal-time logicPetri Net modelAfter hazards identification & analysis, the next step is to specify safety related requirements, i.e., to findA list of undesirable events & the desired system responses to these eventsRole of s/w in managing undesirable events is then indicated
16 The ISO 9000 Quality Standard A quality assurance system may be defined as the organizational structure, responsibilities, procedures, processes, & resources for implementing quality management.ISO 9000 describes a quality assurance system in generic terms that can applied to any business regardless of the products services offered.
17 Getting Certified…To become registered to one of the quality assurance system models contained in ISO 9000, a company’s quality system & operations are scrutinized by third-party auditors for compliance to the standard for effective operation.Upon successful registration, the company is issued a certificate from a registration body represented by the auditors.Semi-annual surveillance audits ensure continued compliance to the standard.
18 ISO 9001:2000ISO 9001:2000 is the quality assurance standard that applies to software engineering.The ISO 9001:2000 standard contains 20 requirements.For a software organization to become registered to this standard, it must establish policies & procedures to address each of these requirementsAs ISO 9001:2000 standard is applicable to all engg. disciplines, a special set of ISO guidelines (ISO ) have been developed for use in software process.
20 The SQA Plan Provides a roadmap for establishing SQA Developed by SQA group (or software team if SQA group does not exist)A standard structure for SQA plans by IEEE recommends the following:Scope & purpose of the plan.Description of all s/w engg. work products that fall within range of SQA.All applicable standards & practices that are applied during the software process.
21 The SQA PlanSQA actions & tasks (including reviews & audits) and their placement throughout the software process.Tools and methods that support SQA actions & tasks.Software configuration management procedures for managing change.Methods for assembling, safeguarding, and maintaining all SQA-related records.Organizational roles and responsibilities relative to product quality