SQE(Lecture 3)

8/6/2019 SQE(Lecture 3)

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Defect Rate and Reliability

Defect Rate: Define as the number of defects perthousand source lines of code or per thousand lines

of code (KLOC or KSLOC) in a given unit of time.

Example: One year after the general availability of the

product in the market place.

Alternatively defect rate can be define as

Defect rate = (Number of defects/OFE) *k.

Where,

OFE= Opportunities for error(Known as risk exposure)

K= Is a constant usually 1,000.


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Software Reliability

The probability of failure free operation of a

computer program in a specified environment for a

specified time.

Example:

Suppose a program X is estimated to have a

reliability of 0.96 over eight elapsed processing

hours.


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If the program X is to be executed 100 times.and required a total ofeight hours of elapsed processing

time.

It is likely to operate correctly (without failure) 96 times.

A simple measure of reliability is mean-time-

between-failure (MTBF).

MTBF= MTTF+MTTRWhere, MTTF= Mean-time-to-failure (Is an inverse of

failure rate)

MTTR=Mean-time-to-repair.


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Prob: A company manufacture resistors that are known tohave an failure rate of 0.15% per 1,000 hours. Find theMTTF for the resistor.

Solu: Failure rate= 0.15% per 1,000 hours.= 0.15%/1000

= 0.0000015.

Therefore MTTF = 1/0.0000015= 666,667 hours.


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Software Availability

Software availability is the probability that a

program is operating according to the requirements at

a given point in time.

Availability= [MTTF/(MTTF+MTTR)]*100%


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Defect Prevention

It is a process to continually improve thedevelopment process.

It is a real time process, integrated into every stage of

the development process.

Defect prevention is based on three simple steps.

a) Analyze defects or errors to trace the

root causes.b) Suggest preventive actions to eliminate

the defect root causes.

c) Implement the preventive actions


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Defect prevention process used by the IBM

communication programming laboratory consist of

the following four key elements.

Causal analysis meeting:Usually two-hour brainstorming session conducted by

technical teams at the end of each stage of the

development process.

Developers analyze defects that occurred in the stage,

trace the root causes of the error, and suggest possible

actions to prevent similar errors from recurring.


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Methods for removing similar defects in a currentproduct are also discuss.

After the meeting, the causal analysis leader records

the data (Defect cause and suggested action) in anaction database for subsequent reporting and tracking.

Action team:

The action team is responsible for screening, prioritizing, and implementing suggested actions

from causal analysis meeting.


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The team uses reports from the action database to

guide its meeting.

Other than implementation, the team is involve infeedback to the organization, report to the

management on the status of its activities and taking

the lead in various aspects of the process.


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Stage kickoff meeting:Conducted at the beginning of the development

process by the technical team.

The emphasis on the technical aspect of thedevelopment process and on quantity.

What is the right product?

How do we thinks more effectively?

What are the tools and methods that can help?


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Action tracking and data collection:

To prevent suggestions from being lost over

time.

To add action implementation, and to enhance

communication among groups.


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DPP can be applied to any development process i.e.

waterfall, prototyping, iterative and spiral.

As long as the defects are recorded, casual analysis

can be performed and preventive actions mapped and

implemented.


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Example of defect prevention

process to the middle segmentof waterfall model.


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SQE(Lecture 3)

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