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Department of Computer & Information SciencesPakistan Institute of Engineering and Applied Sciences
Software Quality
Department of Computer & Information SciencesPakistan Institute of Engineering and Applied Sciences
Lecture 2 Quality Cost Concepts
Umar Faiz http://www.pieas.edu.pk/umarfaiz/cis
Software Quality Umar Faiz http://www.pieas.edu.pk/umarfaiz/cis
Quality Cost Concepts
Software Quality Management
Concerned with ensuring that the required level of quality is achieved in a software product Involves defining appropriate quality standards and procedures and ensuring that these are followed Should aim to develop a ‘quality culture’ where quality is seen as everyone’s ’ responsibilityibilit
Taxonomy of Software Qualities
Runtime System Qualities:
Qualities that can be measured as the system executes. Functionality The ability of the system to do the work for which it was intended. Performance The response time utilization and throughput behavior of theThe response time, utilization, and throughput behavior of the system. Security A measure of system’s ability to resist unauthorized attempts at usage or behavior modification, while still providing service to legitimate users.
Taxonomy of Software Qualities
Runtime System Qualities:
Availability The measure of time that the system is up and running correctly; the length of time between failures and the length of time needed to resume operation after a failure. Usability The ease of use and of training the end users of the system. Sub qualities: learnability, efficiency, affect, helpfulness, control. Interoperability The ability of two or more systems to cooperate at runtime
Taxonomy of Software Qualities
Non‐Runtime System Qualities :
Non‐Runtime System Qualities cannot be measured as the system executes. Modifiability The ease with which a software system can accommodate changes to its software Portability The ability of a system to run under different computing environments. The environment types can be either hardware or software, but is usually a combination of the two.
Taxonomy of Software Qualities
Non‐Runtime System Qualities :
Reusability The degree to which existing applications can be reused in new applications. Integrability The ability to make the separately developed components of the system work correctly together. Testability The ease with which software can be made to demonstrate its faults
Taxonomy of Software Qualities
Non‐Software System Qualities:
Non‐Software System Qualities that influence other quality attributes. Cost and Schedule The cost of the system with respect to time to market, expected project lifetime, and utilization of legacy and COTS systems. Marketability The use of the system with respect to market competition. Appropriateness for Organization Availability of the human input, allocation of expertise, and alignment of team and software structure.
Taxonomy of Software Qualities
Architecture Qualities:
Quality attributes specific to the architecture itself. Conceptual Integrity The integrity of the overall structure that is composed from a number of small architectural structures. Correctness Accountability for satisfying all requirements of the system.
Taxonomy of Software Qualities
Domain Specific Qualities:
Quality attributes found in specific business domains. Sensitivity The degree to which a system component can pick up something being measured. Calibrability Ability of a system to recalibrate itself to some specific working range.
Cost of Quality
What is Cost of Quality?
As defined by Crosby ("Quality Is Free"), Cost Of Quality (COQ) has two main components: Cost Of Conformance Cost Of Non‐Conformance Quality costs represents the difference between the actual cost t off a productd t or servicei andd whath t th th e reducedd d costt would be if there were no possibility of a sub‐standard service
Cost of Quality
Cost Of Conformance
A component of the Cost Of Quality for a work product. Cost of conformance is the total cost of ensuring that a product is of good Quality. It includes costs of Quality Assurance activities: Standards, training, and processes; and costs of Quality Control activities such h as reviews, audits,d inspections, andd testing. COC represents an organization's investment in the quality of its products.
Quality Cost Concepts
Why Cost of Quality?
Quality processes cannot be justified simply because "everyone else is doing them" ‐ but return on quality (ROQ) has dramatic impacts as companies mature. Typically, the cost to eliminate a failure in the customer phase is five times greater than it is at the development or manufacturing phasemanufacturing phase. Effective quality management decreases production costs because the sooner an error is found and corrected, the less costly it will be.
Quality Cost Concepts
Goal of a Quality Cost System
The goal of any quality cost system is to facilitate quality improvement efforts that will lead to operating cost reduction opportunities. The strategy for using quality costs is quite simple: Take direct attack on failure costs in an attempt to drive them to zero Invest in the prevention activities to bring out improvement Total Quality Costs Reduce appraisal costs according to results achieved Continuously evaluate and redirect prevention efforts to gain further improvement.
Quality Cost Concepts
Goal of a Quality Cost System
The strategy is based on the premise that For each failure there is a root cause Causes are preventable Prevention is always cheaper
Taguchi Quality Loss Function
Taguchi Methods
The quality loss function is based on the work of electrical engineer, Genichi Taguchi. Taguchi methods are both a philosophy and a collection of tools used to carry forth that philosophy We cannot reduce cost without affecting quality We can improve quality without increasing cost We can reduce cost by improving quality We can reduce cost by reducing variation. When we do so, performance and quality will automatically improve
Taguchi Quality Loss Function
There are five types of quality characteristics:
Nominal‐the‐best (achieving a desired target value with minimal variation, such as dimension and output voltage) Smaller‐the‐better (minimizing a response, such as shrinkage and wear) Larger‐the‐better (maximizing a response, such as pull‐off ff orce andd t t ensileil strength)t th) Attribute (classifying and/or counting data, such as appearance) Dynamic (response varies depending on input, such as the speed of a fan drive should vary depending on the engine temperature)
Taguchi Quality Loss Function
Nominal‐the‐best
In this approach, the closer to the target value, the better. It does not matter whether the deviation is above or below the target value. Under this approach the deviation is quadratic..
Taguchi Quality Loss Function
Smaller‐the‐better
The smaller is better approach is when a company desires smaller values. As the value gets larger, the loss incurred grows.
Taguchi Quality Loss Function
Larger‐the‐better
This approach is used when a company desires higher values of a characteristic. Two examples given are employee participation and the customer acceptance rate. Under this approach, the larger the characteristic, the smaller the quality loss function.
Taguchi Quality Loss Function
Uses of Quality Loss Function (QLF) Data
- Reduces Costs There are three ways that managers can use QLF to reduce costs. 1. Move the average of the actual distribution closer to the target value. 2. Reduce variability. 3. Do a combination of both.
Taguchi Quality Loss Function
Uses of Quality Loss Function (QLF) Data
- Setting Specific Limits The data from the quality loss function can be used to determine where limits should be set to help minimize losses.
Taguchi Quality Loss Function
Sony Corporation Example
Sony uses the Taguchi model in managing the television sets it produces. The quality characteristic is the color density of the televisions. The Sony engineers set specific limits for color density at a plus or minus tolerance level.
Taguchi Quality Loss Function
Sony Corporation Example
One of Sony’s plants uniformly distributed televisions that fell between the specification limits. The other plant followed a normal distribution with an average near the set target. A comparison of customer responses shows that a higher level of satisfaction was reported on televisions from thelevel of satisfaction was reported on televisions from the second plant. Also, the second plant’s warranty expenses were lower.
Taguchi Quality Loss Function
Sony Corporation Example
The first plant shipped at a zero defect rate, however, the specification limits allowed for too much variation. In the second plant, the limits were smaller and the quality was more consistent.
In conclusion, if companies want to remain competitive, they have to provide quality products. To accomplish this, a company must focus on the reduction of variability of a product’s characteristics around a specific target value.
