Quality of software
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This document discusses software quality and its attributes. It defines software quality as conformance to functional and performance requirements, development standards, and implicit expectations. Problems in ensuring quality include incomplete specifications and tensions between different stakeholder needs. Quality is described using a hierarchical model, with attributes including reliability, efficiency, usability, maintainability, and portability. Internal attributes like correctness, verifiability and understandability contribute to external attributes like reliability, usability and maintainability. Productivity, timeliness and visibility are described as important process quality attributes.
Quality of software
- 1. QUALITY OF SOFTWARE OR SOFTWARE QUALITY PRESENTED BY: PALAK PANDOH
- 2. OVERVIEW OF SOFTWARE QUALITY What is Quality? Quality Attributes
- 3. OVERVIEW OF SOFTWARE QUALITY What is Quality? Quality Attributes
- 4. What is Quality? Software Quality is conformance to: Explicitly stated functional and performance requirements, Explicitly documented development standards, Implicit characteristics that are expected of all professionally developed software.
- 5. Problems with Software Quality Software specifications are usually incomplete and often inconsistent There is tension between: ◦ customer quality requirements (efficiency, reliability, etc.) ◦ developer quality requirements (maintainability, reusability, etc.) Some quality requirements are hard to specify in an unambiguous way ◦ directly measurable qualities (e.g., errors/KLOC), ◦ indirectly measurable qualities (e.g., usability). Quality management is not just about reducing defects!
- 6. OVERVIEW OF SOFTWARE QUALITY What is Quality? Quality Attributes
- 7. Hierarchical Quality Model Define quality via hierarchical quality model, i.e. a number of quality attributes (a.k.a. quality factors, quality aspects, ...) Choose quality attributes (and weights) depending on the project context Software Quality ... Reliability Efficiency Usability Maintainability Portability may be further refined into subattributes Quality attribute
- 8. Quality Attributes Quality attributes apply both to the product and the process. product: delivered to the customer process: produces the software product resources: (both the product and the process require resources) ◦ Underlying assumption: a quality process leads to a quality product (cf. metaphor of manufacturing lines)
- 9. Quality Attributes ... Quality attributes can be external or internal. External: Derived from the relationship between the environment and the system (or the process). (To derive, the system or process must run) ◦ e.g. Reliability, Robustness Internal: Derived immediately from the product or process description (To derive, it is sufficient to have the description) ◦ Underlying assumption: internal quality leads to external quality (cfr. metaphor manufacturing lines) ◦ e.g. Efficiency
- 10. Correctness, Reliability, Robustness Correctness A system is correct if it behaves according to its specification ◦ An absolute property (i.e., a system cannot be “almost correct”) ◦ ... in theory and practice undecidable Reliability The user may rely on the system behaving properly Reliability is the probability that the system will operate as expected over a specified interval ◦ A relative property (a system has a mean time between failure of 3 weeks) Robustness A system is robust if it behaves reasonably even in circumstances that were not specified A vague property (once you specify the abnormal circumstances they become part of the requirements)
- 11. Efficiency, Usability Efficiency (Performance) Use of resources such as computing time, memory ◦ Affects user-friendliness and scalability ◦ Hardware technology changes fast! ◦ First do it, then do it right, then do it fast For process, resources are manpower, time and money ◦ relates to the “productivity” of a process
- 12. Efficiency, Usability ... Usability (User Friendliness, Human Factors) The degree to which the human users find the system (process) both “easy to use” and useful ◦ Depends a lot on the target audience (novices vs. experts) ◦ Often a system has various kinds of users (end-users, operators, installers) ◦ Typically expressed in “amount of time to learn the system”
- 13. Maintainability External product attributes (evolvability also applies to process) Maintainability How easy it is to change a system after its initial release ◦ software entropy maintainability gradually decreases over time
- 14. Maintainability ... Is often refined to ... Repairability How much work is needed to correct a defect Evolvability (Adaptability) How much work is needed to adapt to changing requirements (both system and process) Portability How much work is needed to port to new environment or platforms
- 15. Verifiability, Understandability Internal (and external) product attribute Verifiability How easy it is to verify whether desired attributes are there? ◦ internally: e.g., verify requirements, code inspections ◦ externally: e.g., testing, efficiency Understandability How easy it is to understand the system ◦ internally: contributes to maintainability ◦ externally: contributes to usability
- 16. Productivity, Timeliness, Visibility External process attribute (visibility also internal) Productivity Amount of product produced by a process for a given number of resources ◦ productivity among individuals varies a lot ◦ often: productivity (∑ individuals) < ∑ productivity (individuals)
- 17. Productivity, Timeliness, Visibility ... Timeliness Ability to deliver the product on time ◦ important for marketing (“short time to market”) ◦ often a reason to sacrifice other quality attributes ◦ incremental development may provide an answer Time Function User needs System capability t0 t1 t2 t3 t4 initial redesigndelivery
- 18. Productivity, Timeliness, Visibility ... Visibility (Transparency) Current process steps and project status are accessible ◦ important for management ◦ also deal with staff turn-over
- 19. REFERENCES Software Engineering, I. Sommerville, 7th Edn., 2004. Fundamentals of Software Engineering, C. Ghezzi, M. Jazayeri, D. Mandroli, Prentice-Hall 1991
- 20. THANK YOU © Oscar Nierstrasz LECTURE TITLE 20
Editor's Notes
- #5: Pressman ch 17; Sommerville ch 30