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Software Testing and Quality Assurance

Lecture 38 – Software Quality Assurance

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Lecture Objectives Software Safety

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Software Safety Safety in systems involving software is

becoming important. For example, Computer Aided Dispatch Systems (CAD); Electronic Flight Control Systems (EFCS). Train Protection Systems; Chemical Plant control systems.

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Software Safety

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Software Safety We wish to avoid in engineering and

operating our platforms is Accidents.

The system that we build must avoid the hazards that lead to accidents.

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Software Safety Accident – an event of sequence of

events leading to harm; that is, death, injury, environmental damage or financial loss.

Hazard – a physical situation or state of the platform that can lead to an accident.

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Software Safety To understand the safely of a system

Understand how they can fail. Investigate accidents and accident

sequences To understand the sequence of events

leading to the accident and to try and determine which subsystem failed.

Accidents are usually caused by

combination of failures and circumstances.

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Software Safety Hazards are really accidents “waiting to

happen”. They are the pre-conditions for an accident.

In hazard identification We are concerned with thinking about the safety of

the personal or the platform. Start from known accidents or consider possible

accidents and work back to hazards. Brainstorming exercise.

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Software safety is not just software Reliability

Failure is key to understanding software reliability. Failure is deviation from the specified

behavior of the system. For safely,

it is always a deviation from the intended behavior; and where the deviation can lead to harm or damage.

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Software safety is not just software Reliability

In normal usage of word failure A system may be unreliable but still safe; It may be completely reliable but totally

unsafe.

What failure of the system lead to an unsafe system?

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Safety Engineering Approach Hazard analysis technique to determine

the safety aspects of the system Early in the development process, then Monitoring safety throughout the product

development process; and Ensuring that there is enough evidence to

build a safety case at the end of the product development process.

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Key points Accidents and Hazards

Accidents are usually caused by combination of failures and circumstances.

Hazards are really accidents “waiting to happen”.

Safety Engineering Approach