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.

Prevention Costs

• quality planning
• formal technical reviews
• test equipment
• training

Appraisal Costs-- include activities to gain insight into product condition the "first time through" each process

• in-process and interprocess inspection
• equipment calibration and maintenance
• testing

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.

• rework
• repair
• failure mode analysis

External Failure Costs-- costs associated with defects found after the product has been shipped to the customer.

• complaint resolution
• product return and replacement
• help line support
• warranty work

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:

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:

• between three and five people (typically) should be involved in the review
• advance preparation should occur but should require no more than two hours of work for each person
• the duration of the review meeting should be less than two hours

At the end of the review meeting, the participants must decide whether to:

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:

• Review the product, not the producer
• Set an agenda, and maintain it
• Limit debate and rebuttal
• Enunciate problem areas, but don't attempt to solve every problem noted
• Take written notes
• Limit the number of participants and insist upon advance preparation
• Develop a checklist for each product that is likely to be reviewed
• Allocate resources and time schedule for FTR's
• Conduct meaningful training for all reviewers
• Review your early 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:

Suggested Links for Further Readings

Last Modified: Wednesday, 13-Jan-99 9:10:00 CDT
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Comments: reithel@bus.olemiss.edu