Two Kinds of Quality
Quality of Design-- refers to the characteristics that designers specify for an item. The grade of materials, tolerances, and performance specifications all contribute to the quality of design. The design quality of a product increases as higher-graded materials are used and tighter tolerances and greater levels of performance are specified.
Quality of Conformance-- is the degree to which the design specifications are followed during manufacturing. The greater the degree of conformance, the higher the level of quality of conformance.
Kinds of Costs
Costs of quality include all costs incurred in the pursuit of quality or in performing quality related activities, and may be subdivided into costs associated with prevention, appraisal, and failure.
Appraisal Costs-- include activities to gain insight into product condition the "first time through" each process
Failure Costs-- costs that would disappear if no defects appeared before shipping a product to customers. Failure costs should be subdivided into internal and external failure costs.
Internal Failure Costs-- costs incurred when we detect an error in our product prior to shipment.
External Failure Costs-- costs associated with defects found after the product has been shipped to the customer.
Defect Amplification Model
The Defect Amplification Model can be used to illustrate the generation and detection of errors during preliminary design, detail design, and coding steps of the software engineering process and adopts a "pay now or pay much more later" approach. The model suggests that during the software development step, errors may be inadvertently generated. Review may not uncover the newly generated errors and errors from previous steps, resulting in some number of error that are passed through to the next step. In some cases, the errors passed through from previous steps are amplified by current work, causing the problem to be much greater in the next step.
Formal Technical Review-- A formal technical review (FTR) is a software quality assurance activity that is performed by software engineers. The objectives of FTR are:1. to uncover error in function, logic, or implementation for any representation of the software
The FTR is actually a class of reviews that include walkthroughs, inspections, round-robin reviews, and other small group technical assessments of software. In addition to the above objectives, FTR serves as a training ground, enabling junior engineers to observe different approaches to software analysis, design, and implementation.
Regardless of the format, every review meeting should abide by the following constraints:
At the end of the review meeting, the participants must decide whether to:1. accept the work product without further modification
It is important to establish guidelines for the conduct of formal technical reviews in advance. The following represents a minimum set of guidelines for formal technical reviews:
Mean Time Between Failure (MTBF)
In hardware, failures due to physical wear are more common than design related failure. However, the opposite is true for software (actually, all software failures can be traced to design or implementation problems). Keeping this in mind, there is still debate over the relationship between key concepts in hardware reliability and their applicability to software failure.
Although no definite link has been established, it is important to learn key concepts that apply to both system elements. A simple measure of reliability is mean time between failure (MTBF), where
MTBF = MTTF (mean time to failure) + MTTR(mean time to repair)
ISO 9000 describes quality assurance elements and standards in generic terms that can be applied to any business.
ISO 9001 is the quality assurance standard that applies to software engineering. The 20 requirements delineated by ISO 9001 address the following topics:
|1. Management responsibility||11. Control of inspection, measuring, and test equipment|
|2. Quality system||12. Inspection and test status|
|3. Contract review||13. Control of nonconforming product|
|4. Design control||14. Corrective and preventive action|
|5. Document and data control||15. Handling, storage, packaging, preservation, and delivery|
|6. Purchasing||16. Control of quality records|
|7. Control of customer supplied product||17. Internal quality audits|
|8. Product identification and traceability||18. Training|
|9. Process control||19. Servicing|
|10. Inspection and testing||20. Statistical techniques|
Suggested Links for Further ReadingsGeneral information on SQA (American Society for Quality Control)
Last Modified: Wednesday, 13-Jan-99 9:10:00 CDT
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