1. Real-World Challenges in Building Accurate Software Fault Prediction Models DR. ÇA Ğ ATAY ÇATAL TUBITAK (Research Council of TURKEY) Predictive Modelling and Search Based Software Engineering, London, UK, 24-25 October 2011

2. 2 Outline Introduction Dependable Software Systems Motivation Challenging Issues Fault prediction with no fault data Fault prediction with limited fault data Noise detection on measurement datasets Practical tools (Eclipse plug-in) Cross company vs. Within-company fault prediction Our Models A Systematic Review Study Conclusion

3. Dependable Systems Are we successful in building dependable software systems? Safety (not being harmful for environment) Security(ability to protect the privacy) Reliability(ability to perform its function for a period of time) Availability (ability to serve whenever needed) 3

4. 4 1. BRITISH ATM PAYS DOUBLE ! 19 March 2008 ATM pays out double the amount withdrawn Dozens of customers lined up in front of ATM This continued until ATM ran out of money at 8 p.m. Hull, England

5. A Generous British ATM... 5 A Sainsburrys spokesman said We do not know how much the machine paid out at the moment but the matter is under investigation A customer said I joined the queue and when I finally got to the front I drew out 200 pound but it gave me 400 pound. The statement said I only drew out 200 pound. I dont know whether I will have to pay it back The police said Those who benefited could face charges, but only if the company administering the machine complained.

6. 2. ATM Pays Out Double the Cash, 16 January 2009 6

7. 3. Tesco machine pays double, 18 August 2009 7

8. 4. Dundee cash machine, 20 January 2011 8

9. But what happens if an ATM malfunctions and pays out less than you asked for? We need dependable systems ! 9

10. 10 Motivation Project Managers ask several questions: How can I get the code into production faster? What code should we refactor? How should I best assign my limited resources to different projects? How do I know if code is getting better or worse as time goes on? Baseline Code Analysis Using McCabe IQ Software Metrics Software Fault Prediction

11. 11 Example: gcc project /trunk/gcc/fold-const.c http://gcc.gnu.org/viewcvs/trunk/gcc/fold- const.c?revision=135517&view=markup http://gcc.gnu.org/viewcvs/trunk/gcc/fold- const.c?revision=135517&view=markup fold_binarys CC value is 1159 ! Security problems or faults can occur

12. 12 Vulnerability Report – Fold_Binary Method http://vulnerabilities.aspcode.net/14389/fold+binary+in+fold+const+c+in+GNU+Compiler+Col.aspx

13. CHAPTER 2: Challenging Issues

14. 14 Software Fault Prediction Modeling Previous Version Training Learnt Hypothesis Predict Faults Software Metrics Software Metrics Current Project Known Fault Data Unknown Faul t Data

15. 15 1. No Fault Data Software Metrics Software Metrics Unknown Fault Data Learnt Hypothesis Previous Version Current Project Training Predict Faults Unknown Fault Data * How does the software quality assurance team predict the software quality based on only the recorded software metrics? - A new project type for organization - No quality measurement have not been collected * Supervised learning approach cannot be taken

16. 16 2. Limited Fault Data Software Metrics Software Metrics Unknown Fault Data Learnt Hypothesis Previous Version Current Project Training Predict Faults Known Fault Data Unknown Fault Data * During decentralized software development, some companies may not collect fault data for their components * Execution cost of data collection tools may be expensive * Company may not collect fault data for a version due to the lack of budget - Can we learn both from labeled and unlabeled data?

17. 17 3. Noise Detection Noisy modules degrades the performance of machine learning based fault prediction models Attribute Noise Class Noise Class noise impact classifiers more severely as compared to attribute noise We need to identify noisy modules if they exist Some cases: Developers may not report the faults Data entry and data collection errors

18. 18 4. Practical Tools Earliest Work, Porter and Selby, 1990.... Logistic Regression (Khoshgoftaar et al., 1999) Decision Trees (Gokhale et al., 1997) Neural Networks (Khoshgoftaar et al., 1995) Fuzzy Logic (Xu, 2001) Genetic Programming (Evett et al., 1998) Case-Based Reasoning (Khoshgoftaar et al., 1997) Pareto Classification (Ebert, 1996) Discriminant Analysis (Ohlsson et al., 1998) Naive Bayes (Menzies et al., 2008)... Hundreds of research papers but lacking of practical tools…

19. 19 5. Cross-Project vs. Within-Company Fault Prediction Can we use cross-company (CC) data and predict the fault-proneness of program modules in the absence of fault labels?

20. CHAPTER 3: Models we built...

21. 1. No Fault Data 21

22. 22 1. No Fault Data Problem- Literature Zhong et al., 2004, Clustering and Expert based Approach K-means and Neural Gas algorithms Mean vector and several statistical data such as min., max. Dependent on the capability of the expert Zhong, S., T. M. Khoshgoftaar, and N. Seliya, Unsupervised Learning for Expert-based Software Quality Estimation, Proceedings of the 8th Intl. Symp. on High Assurance Systems Engineering, Tampa, FL, 2004, pp. 149-155.

23. 23 1. No Fault Data Problem 1. Our technique first applies X-means clustering method to cluster modules and identifies the best cluster number. 2. The mean vector of each cluster is checked against the metrics thresholds vector. A cluster is predicted as fault-prone if at least one metric of the mean vector is higher than the threshold value of that metric. [LOC, CC, UOp, UOpnd, TOp, TOpnd] [65, 10, 25, 40, 125, 70] (Integrated Software Metrics (ISM) document)

24. 24 Datasets from Turkish white-goods manufacturer Effective results are achieved No expert opinion Identification of threshold vector is difficult

25. 2. Limited Fault Data Problem 25

26. 26 2. Limited Fault Data Problem We simulated small labeled-large unlabeled data problem with 5%, 10%, and 20% rates and evaluated the performance of each classifier under these circumstances. Naive Bayes algorithm, even if it is a supervised learning approach, works best for small datasets YATSI (Yet Another Two Stage Idea) improves the performance of Naive Bayes algorithm for large datasets if the dataset does not consist of noisy modules We suggest Naive Bayes for limited fault data problem as well

27. 3. Noise Detection 27

28. 28 3. Noise Detection Our hypothesis: A data object that has a non-faulty class label is considered a noisy instance if the majority of the software metric values exceed their corresponding threshold values. A data object that has a faulty class label is considered a noisy instance if all of the metric values are below their corresponding threshold values. How to calculate software metrics threshold values? R. Shatnawi, W. Li, J. Swain, T. Newman, Finding software metrics threshold values using ROC curves, Journal of Software Maintenance and Evolution: Research and Practice 22 (1) (2010) 1–16.

29. 29 How to Calculate Threshold Values The interval for the candidate threshold values is between the minimum and maximum value of that metric in the dataset. Shatnawi et al. (2010) stated that they chose the candidate threshold value that has the maximum value for both sensitivity and specificity, but such a candidate threshold may not always exist. We calculated the AUC of the ROC curve that passes through three points, i.e., (0, 0), (1, 1), and (PD, PF), and we chose the threshold value that maximizes the AUC.

30. 30

31. 31

32. 4. Practical Tools 32

33. 33 4. Eclipse based Plug-in (RUBY)

34. Sample User Interfaces - Features 34

35. Result Views 35

36. 5. Cross-Project Fault Prediction 36

37. 37 5. Cross-Project Fault Prediction We developed models based on software metrics threshold values If majority of software metrics thresholds values are exceeded, the label of the module is faulty Otherwise, non-faulty label is assigned Threshold values are calculated from the other projects (cross-company)

38. 38

39. AUC, PD, PF 39

40. Results Case studies showed that the use of cross-company data is useful for building fault predictors in the absence of fault labels and remarkable results are achieved. Our threshold-based fault prediction technique achieved larger PD (but larger PF) value than Naive Bayes based approach. For mission critical applications, PD values are more important than PF values because all of the faults should be removed before deployment. In summary, we showed that cross-company dataset is useful. 40

41. 4. Systematic Review 41

42. 42 A Systematic Review Study 74 papers published between 1990 and 2007 27 journal papers 47 conference papers We report distributions before and after 2005, since that was the year that the PROMISE repository was established.

43. Results The journals that published more than two fault model papers are: IEEE Transaction of Software Engineering (9); Software Quality Journal (4); Journal of Systems and Software (3); Empirical Software Engineering (3) 14% of papers were published before 2000 and 86% after. Types of data sets used by authors were: private (60%), partial (8%), public (31%), unknown (1%). Partial means data from open source projects that have not been circulated. Since 2005 the proportion of private datasets has reduced to 31%, the proportion of public data sets has increased to 52%. There are 14% partial datasets and 3% unknown. 43

44. Results (contd) Data analysis methods are machine learning (59%), statistics (22%), statistics and machine learning (18%) and statistics and expert opinion (1%). After 2005 the distribution of methods is machine learning (66%), statistics (14%), statistics and machine learning (17%) and statistics and expert opinion (3%). 60% of papers used method level metrics, 24% used class level metrics, 10% were file level metrics, other categories less than 5%. 2005, 53% were method level, 24% were class level and 17% were file level (others less than 3%). 44

45. Suggestions More studies should use class-level metrics to support early prediction. Fault studies should use public datasets to ensure results can be repeatable and verifiable. Researchers should increase usage of machine learning techniques. 45

46. 46 Conclusion & Future Work Software fault prediction is still challenging and quite useful We need practical tools Prediction models can be used to predict vulnerability-prone modules Challenges How to make fault prediction work across projects ? How to build models when there is no fault data? How to build models when there is very limited fault data? How to remove noisy modules from datasets?

47. 47 THANK YOU Cagatay CATAL, Ph.D. cagatay.catal@bte.tubitak.gov.tr www.cagataycatal.com