1. Software Defects

2. What leads to what? ERROR FAULT FAILURE Observed by Introduced by

3. Product defects Defect = Fault or Failure i.e. Defect is either the cause of failure or the failure itself (as the effect of the fault)

4. What constitutes failure 1.Failing product deadlines 2.Failing product budget (cost) 3.Failing product quality

5. Defect removal costs (taken and adapted from software development data in Germany in the 90s) 40% 50% 25% KLm90 KLm50 KLm25 Lm200 10% Fault Source Fault Detection Per fault Cost 3% 5% 7% 10% Require- ments DesignCoding Component testing Integration testing System use

6. Availability Availability is the probability of getting system service at any given point in time. Availability = sys_uptime/(sys_uptime + sys_downtime) -or- Failure_hazard/(failure_hazard + repair_hazard) -or- z(t)/(z(t) + r(t))

7. Reliability Many defs… Musa et al.: “The probability that a system, or a capability of a system, functions without failure for a specific time or number of natural units” Sommerville: “The probability of failure- free operation for a specified environment for a given purpose”

8. Reliability facts 1.The more failures are observed and corrected the more reliability improves  System usage must be clearly modelled 2.Reliability depends on how a system is used  Reliability growth must be modelled 3.System failures occur randomly  Failure must be modelled probabilistically

9. H/W vs S/W Failure Rates Failure rate time Failure rate

10. Zero-Failure Testing (Motorola™) [ln(failures/(0.5 + failures))] * (hours-to-last-failure) ln[(0.5 + failures)/(test-failures + failures)] Based on the Brettschneider failure rate function (1989) [ ae -bt ]

11. Some measures MTTF: Mean time to failure The time between the system starts and failures MTTR: Mean time to repair The time a system is in repair MTBF: Mean time between failures The time between subsequent system failures

12. Diagrammatically x1x1 x2x2 x3x3 x4x4 x5x5 y1y1 y2y2 y3y3 y4y4 y4y4 z1z1 z2z2 z3z3 z4z4 MTTF = avg(x n )MTBF = MTTF + MTTR MTTR = avg(y n )A = MTBF/(1 - MTBF) MTBF = avg(z n )M = 1/(1 + MTTR) R = MTTF/(1 + MTTF)