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Software Quality Assurance



Introduction to Software QualityAssurance

Left: The Ariane 501rocket·s guidance computer

software just threw anunchecked exception.

[Image source: European

Space Agency]



Ob jectives

What is Software Quality? Why is Software Quality important?

What is Software Quality Assurance ?

Software Quality factors

Elements of Software Quality Assurance

Development and quality plans

Process Maturity models



What is Software ?

According to the IEEE Software is:

² Computer programs, procedures, and possiblyassociated documentation and data pertainingto the operation of a computer system.



Software Errors, software faults andsoftware failures

Bug/defect/fault consequence of a human error.² Results in non-conformance to requirements.

² Manifests as failure in running software.

Software development process

software fault

softwarefailure

softwareerror



Nine Causes of Software Errors

Faulty requirements definition. Client-developer communication failures.

Deliberate deviations from software requirements.

Logical design errors.

Coding errors.

Non-compliance with documentation and coding instructions.

Shortcomings of the testing process.

User interface and procedure errors.

Documentation errors.



When are defects introduced?

The majority of defects are introduced in earlierphases.

² Requirements are the top factor in a project·ssuccess or failure.

Phase % ofdefects

% effort tofix

Requirements 56 82

Design 27 13

Code 7 1Other 10 4



Cost of fixing defects

Relative cost of fixing defects² benchmark: cost at requirements phase = 1

Phase found Cost ratio

Requirements 1Design 3 ² 5

Coding 10

Unit / integration testing 15 - 40

System / acceptance testing 30 ² 70

Production 40 - 1000



What is Software Quality ?

Conformance to requirements. Narrowest sense of software quality.

² Lack of bugs.

² Low defect rate (# of defects/size unit)² High reliability (number of failures per n hours

of operation).

² Mean Time To Failure (MTTF): probability of

failure-free operation in a specified time.




According to the IEEE: Software quality is:

1. The degree to which a system, component, orprocess meets specified requirements.

2. The degree to which a system, component, orprocess meets customer or user needs orexpectations.




According to Pressman: Software quality is:

Conformance to explicitly stated functional and

performance requirements, explicitly documenteddevelopment standards, and implicitcharacteristics that are expected of allprofessionally developed software.



Importance of Software Quality

Software is a major component of computer systems (ab

out80% of the cost) ² used for

² Communication (e.g. phone system, email system).

² Health monitoring.

² Transportation (e.g. automobile, aeronautics).

² Economic exchanges (e.g. e-commerce).² Entertainment.

² etc.

Software defects are extremely costly in term of

² money² reputation

² loss of life




Notable consequences of software errors:

² 1988 shooting down of Airbus 320 b y the USS Vincennes -cryptic and misleading output displayed b y tracking software.

² 1991 patriot missile failure - inaccurate calculation of time dueto computer arithmetic errors.

² London Ambulance Service Computer Aided Dispatch System ²

several deaths.² On June 3, 1980, the North American Aerospace Defense

Command (NORAD) reported that the U.S. was under missileattack.

² First operational launch attempt of the space shuttle, whosereal-time operating software consists of about 500,000 lines of

code, failed - synchronization problem among its flight-controlcomputers.

² 9 hour breakdown of AT&T's long-distance telephone network -caused b y an untested code patch.




Ariane 5 explosion June 4, 1996² First flight of the European Ariane 5 launcher crashed

about 40 seconds after takeoff.

² Cost was about half a billion dollars.

² Explosion was the result of a software error:

² Uncaught exception due to floating-point error:conversion from a 64-bit integer to a 16-bit signedinteger applied to a larger than expected number.

² Module was re-used without proper testing fromAriane 4 .

² Error was not supposed to happen with Ariane 4.

² No exception handler.




Mars Climate Orbiter - September 23, 1999² Mars Climate Orbiter, disappeared as it began

to orbit Mars.

² Cost about $US 125-million.

² Failure due to error in a transfer ofinformation between a team in Colorado and ateam in California.

² One team used imperial units (e.g., inches,

feet and pounds) while the other used metricunits for a key spacecraft operation.




Mars Polar Lander - December, 1999² Mars Polar Lander, disappeared during landing

on Mars.

² Failure most likely due to unexpected setting of

a single data bit.² Defect not caught b y testing.

² Independent teams tested separate aspects.




Internet viruses and worms² Blaster worm (US$ 525 million)

² Sobig.F (US$ 500 million ² 1 billion)

Exploit well known software vulnerabilities

² Software developers do not devote enougheffort to applying lessons learned about thecauses of vulnerabilities.

² Same types of vulnerabilities continue to be

seen in newer versions of products that were inearlier versions.

Usability problems




Monetary impact of poor software quality(Standish group - 1995)

175,000 software projects/year - Average Costper project

² Large companies - $US 2,322,000

² Medium companies - $US 1,331,000

² Small companies - $US 434,000

31.1% of projects canceled before completed.² cost $81 billion




52.7% of projects exceed theirbudget - costing189% of original estimates.

² cost $59 billion

16.2% of software projects completed on-time and

on-budget (9% for larger companies). Large companies - delivered systems have

approximately only 42% of originally-proposedfeatures and functions.

78.4% of smaller companies projects get deployedwith at least 74.2% of their original features andfunctions.

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