Software%20Defects

1
Software Defects
2
What leads to what?
Introduced by
ERROR
Observed by
FAULT
FAILURE
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)
Require- ments
Design
Coding
Component testing
Integration testing
System use
50
Fault Source
40
10
50
Fault Detection
10
7
25
5
3
KLm90
Per fault Cost
KLm50
KLm25
Lm200
Lm200
Lm200
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

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Reliability facts

• The more failures are observed and corrected the
  more reliability improves
• ? System usage must be clearly modelled
• Reliability depends on how a system is used
• ? Reliability growth must be modelled
• System failures occur randomly
• ? Failure must be modelled probabilistically

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

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Diagrammatically
x1
x2
x3
x4
x5
y1
y2
y3
y4
y4
z1
z2
z3
z4
MTTF avg(xn) MTBF MTTF MTTR MTTR
avg(yn) A MTBF/(1 - MTBF) MTBF avg(zn) M
1/(1 MTTR) R MTTF/(1 MTTF)