Lecture08
- 1. Disciplined Software Engineering Lecture #8 Software Engineering Institute Carnegie Mellon University Pittsburgh, PA 15213 Sponsored by the U.S. Department of Defense
- 2. Lecture Overview What is quality? product and process quality quality economics The quality strategy characterizing a process benchmarking a process Yield management defect removal defect prevention
- 3. What is Software Quality? Basic definition meeting the users’ needs needs, not wants true functional needs are often unknowable There is a hierarchy of needs do the required tasks meet performance requirements be usable and convenient be economical and timely be dependable and reliable
- 4. Dependable and Reliable To be used, the software must install quickly and easily run consistently properly handle normal and abnormal cases not do destructive or unexpected things be essentially bug free The latent bugs must be operationally insignificant not be destructive rarely be evident
- 5. A Quality Process Produces quality products Meets its users needs You are the user of the PSP process Your customers are your management peers and associates product’s users
- 6. The PSP Quality Focus - 1 In this course defects are the basic quality measure. Note that bugs are not important to the user as long as they do not affect operations cause inconvenience cost time or money cause loss of confidence in the program’s results
- 7. The PSP Quality Focus - 2 The defect content of software products must first be managed before other more important quality issues can be addressed. Current software processes manage defects so poorly that little if any time is available for such important software quality issues as installability safety performance recovery usability, etc.
- 8. The PSP Quality Focus - 3 Low defect content is an essential prerequisite to a quality software process. Low defect content can best be achieved at the PSP level. This is where the defects are injected and this is where the engineers should remove them determine their causes learn to prevent them
- 9. Tests and Inspections - 1 Without inspections and a 50,000 LOC system 25+ defects/KLOC at test entry that is 1250 defects at the typical 10+ hours per defect, that is 12,500+ programmer hours that is 6 programmer years If properly planned, these tests could be done in 12 to 15 months. If unplanned, testing could take two years or more.
- 10. Tests and Inspections - 2 With inspections and a 50,000 LOC system inspections take about 10 programmer hours per 250 LOC, or about 2,000 hours this is 1 programmer year if done well, inspections can remove about 80% of the defects This means, 250 defects would be left for test this would take about 2,500 hours or a saving of 8,000 hours or a saving of 4 programmer years
- 11. Tests and Inspections - 3 With the PSP code quality will be sharply improved several thousand hours could probably be saved Inspection should still be done the inspection focus should be on design The principal advantages are improved product quality a more predictable schedule time to focus on the important quality issues
- 12. Some Fix Time Data Some typical fix time ratios IBM rules of thumb - coding: 1.5; testing: 60; usage: 100 Boehm - design: 1; development test: 15 to 40; acceptance test: 30 to 70; operation: 40 to 1000 Remus - design: 1, code: 20, test: 82 Ackerman - test: 2 - 10 times inspection time Russell - inspection: 1, test: 2 to 4, use: 33 PSP research - unit test takes 12 times longer than code review to find and fix defects
- 13. The Cost of Quality (COQ) - 1 COQ is a way to measure process quality. COQ has the following components failure costs appraisal costs prevention costs
- 14. The Cost of Quality (COQ) - 2 Failure costs repair, rework, and scrap in PSP, failure costs include all compile and test time Appraisal costs costs of inspecting for defects in PSP, appraisal costs include all design and code review time
- 15. The Cost of Quality (COQ) - 3 Prevention costs finding and resolving defect causes generally handled before projects start should typically be a process and not a project activity In the PSP, examples of prevention costs are formal specification or design work prototyping process analysis and improvement
- 16. The Cost of Quality (COQ) - 4 A useful COQ measure is the ratio of appraisal to failure costs (A/FR). This is (appraisal COQ)/(failure COQ) A/FR experience the A/FR measure is not widely used if measured, most software organizations would be near zero in the PSP, A/FR should exceed 2.0 high A/FR is associated with low numbers of test defects and high product quality
- 17. The Quality Strategy - 1 Identify your PSP quality objectives, i.e. removing all defects before the first compile achieving high productivity producing accurate plans Establish PSP process quality measures, i.e. overall process yield COQ appraisal vs. failure costs - A/FR LOC reviewed per hour Cost performance index - CPI
- 18. The Quality Strategy - 2 Examine the projects you have completed determine their ratings on these measures see what behaviors impacted these results Based on these data, identify the most effective practices for your work. Incorporate these practices in your process process scripts checklists forms
- 19. The Quality Strategy - 3 Identify measures that will reasonably predict process quality establish these as control variables set specifications for these variables Track your performance against these specifications. Track your process to determine if and when to change the specifications actions to take to improve the process further
- 20. Process Benchmarking A method for tracking process improvement should consider quality and productivity provide means for comparing processes used by different people or organizations be insensitive to project specifics Industrial process benchmarking typically deals with the ability of the process to produce products within specifications withstand drift and perturbation
- 21. Software Benchmarking At present, software benchmarking techniques are process dependant. They are still useful, however as long as we establish objective measures track them over time use them for improving the process for which they are designed Comparisons should not be made among individuals or organizations using process sensitive benchmarks.
- 22. Using Software Benchmarks Establish a consistent set of measures for evaluating your process performance take these measures on every project compare individual projects to determine trends or problems Establish and track short-term improvement goals against these measures. Establish and track long-term improvement goals against these measures.
- 23. Benchmarking Data The following data are from various of the students in the PSP course at Carnegie Mellon University in the Spring of 1994. The data are yield by project yield vs A/FR A/FR vs test defects productivity by project yield vs productivity A/FR vs productivity
- 24. class max. class min.
- 25. class max. class min.
- 29. Yield vs. A/FR Conclusions Yield and A/FR are closely related for these students there is considerable variation among students High A/FR ratios appear to lead to higher yields 70+% yields not achieved without A/FRs near 1.0 or above high A/FR does not guarantee high yield - the appraisal time must be spent effectively
- 33. Test Defects vs. A/FR Defects are reduced by high A/FR ratios for all students. To get very low numbers of test defects, A/FR values of above 2.0 appear required. With A/FRs between 1.0 and 2.0, low test defects are occasionally obtained. With an A/FR of below 1.0, low test defects are rare.
- 34. class max. class min.
- 35. class max. class min.
- 42. Yield and A/FR vs. Productivity Productivity has considerable variation among individuals. In some cases, high yield produces higher productivity but in others it does not. High A/FR also sometimes results in high productivity and sometimes not. Clearly, yield and A/FR are somewhat independent of productivity.
- 43. Benchmark Conclusions It is desirable to have high values for yield A/FR productivity Since yield and A/FR are closely related, a yield vs A/FR benchmarking chart would not be useful. Yield vs. productivity or A/FR vs. productivity would likely be useful benchmarking charts.
- 44. Defect Removal Strategies - 1 Focus inspections and reviews on specialties HLD reviews - structural issues DLD reviews - logic correctness code reviews - implementation details To save time, do not address system issues in DLD design issues in code reviews Do the reviews thoroughly the first time and then trust them.
- 45. Defect Removal Strategies - 2 Do thorough unit tests check all parameters at normal values, at limits, and outside limit values check all loops and recursions for normal and abnormal termination check all dependencies among procedures and objects Then do thorough system level testing integration system regression
- 46. Defect Prevention Defect prevention is important because it is always expensive to find defects if the defects can be prevented, these costs are avoided the defect prevention analysis costs are incurred once but save time on every project Defect prevention should follow an orderly strategy and a defined process. For the PSP, defect prevention actions include improved design methods and prototyping.
- 47. Defect Prevention Strategy - 1 Set priorities for the defects types that are the most frequently found defects troublesome defects easily prevented defects
- 48. Defect Prevention Strategy - 2 The defect prevention process has the following steps: 1. follow an established schedule 2. select one or two defect types for initial action 3. track and evaluate the effectiveness of the defect prevention actions 4. make needed adjustments and continue
- 49. Defect Prevention Strategy - 3 In setting your initial priorities, consider the defect types most frequently found in integration and system test. Use PSP data to pick one or two defect types for initial action. Don’t just try harder, establish explicit prevention actions. Incorporate these actions in your process scripts, checklists, and forms.
- 50. Assignment #8 Read chapter 9 of the text Using PSP2, write program 7A to calculate the correlation between two series of numbers and calculate the significance of that correlation. Use program 5A to calculate the values of the t distribution. Consult Appendix C for the PSP2 description and Appendix D for program 7A specifications.
- 51. The Correlation The formula for calculating the correlation coefficient r is Where x and y are the two paired sets of data n is the number of their members
- 52. Correlation Significance The formula for calculating the correlation significance is Where r is the correlation use program 5A to calculate the value of p for the value t from the two-tailed t distribution the significance is indicated by 1 - p > 0.2 is not significant, < 0.05 is significant
- 53. Messages to Remember from Lecture 8 - 1 1. Software quality starts with defects. 2. If defects are not managed, more important quality issues cannot be adequately addressed.
- 54. Messages to Remember from Lecture 8 - 2 3. The most effective way to manage defects is with the individual software engineer 4. If you don’t eliminate your own defects, they will be much more expensive and time consuming to remove later.