Software quality assurance

Introduction
Software EngineeringSoftware Engineering
a “quality” focus
process model
methods
tools
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Introduction
• Quality is defined as a characteristics or attributes of
something where as attributes refer to measurable
characteristics-things that we are able to compare to
known standards.
• There are two kinds of quality:
– Quality of design refers to the characteristics that designers
specify for an item. The grade of materials, tolerances and
performance specifications all contribute to the quality of
design.
– Quality of conformance is the degree to which the design
specifications are followed during manufacturing. Greater the
degree of conformance, the higher is the level of quality of
conformance
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• Software quality is defined as conformance to explicitly stated
functional and performance requirements, explicitly
documented development standards, and implicit
characteristics that are expected of all professionally
developed software.
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• In software development, quality of design encompasses
requirements, specifications, and the design of the system
where as quality of conformance is an issue focused primarily
on implementation.
• If the implementation follows the design and the resulting
system meets its requirements and performance goals,
conformance quality is high.
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Software Quality Assurance(SQA)
• Software quality assurance (SQA) is an umbrella activity that
is applied throughout the software process.
• SQA encompasses:
– A quality management approach
– Effective software engineering technology
– Formal technical reviews
– Multi-tier testing strategy
– Control of software documentation and the changes made
to it
– A procedure to ensure compliance with software
development standards
– Measurement and reporting mechanism
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SQA Activities
• SQA is composed of a variety of tasks associated with two
different constituencies- the software engineer who do
technical work and an SQA group that has responsibility for
quality assurance planning, oversight , record keeping analysis
and reporting.
• The charter of the SQA group is to assist software team in
achieving a high quality end product.
• The SEI recommends a set of SQA activities that address
quality assurance.
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Activities…
• Prepare an SQA plan for a project
– The plan is developed during project planning and is reviewed
by all interested parties. SQA activities performed by the
software engineering team and the SQA team group are
governed by the plan. The plan identifies:
– Evaluations to be performed
– Audits and reviews to be performed
– Standards that are applicable to the project
– Procedures for error reporting and tracking
– Documents to be produced by the SQA team
– Amount of feedback provided to the software project team
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• Participates in the development of the project’s software
process description
– The software team selects a process for the work to be
performed
– The SQA reviews the process description for compliance
with organization policy, internal software standards,
externally imposed standards and other parts of software
project plan.
• Reviews software engineering activities to verify
compliances with defined software process
– The SQA group identifies, documents and track deviations
from the process and verifies that corrections have been
made.
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• Audits designated software work products to verify
compliance with those defined as part of the
software process
– The SQA reviews selected work products, identifies,
documents and track deviations; verifies that correction
have been made; and periodically reports the results of its
works to the project manager
• Ensures that deviations in software work and work
products are documented and handled according to a
documented procedures
• Records any noncompliance and reports to senior
management
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Quality Concepts
• Concerned with ensuring that the required level of quality
is achieved in a software product.
• Three principal concerns:
– At the organizational level, quality management is concerned
with establishing a framework of organizational processes and
standards that will lead to high-quality software.
– At the project level, quality management involves the
application of specific quality processes and checking that these
planned processes have been followed.
– At the project level, quality management is also concerned with
establishing a quality plan for a project. The quality plan should
set out the quality goals for the project and define what
processes and standards are to be used.
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• Examples:
– No two products are similar
– All engineered and manufactured parts exhibit variation.
– Variation control is the heart of quality control.
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Software Reviews
• Software reviews are a filter for the software engineering
process.
• Reviews are applied at various points during software
development and serve to uncover errors and defects that can
then be removed.
• Software reviews purify the software engineering activities
that we have called analysis , design and coding.
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• A review is a way of using the diversity of a group of people
to:
– Point out needed improvements in the product of a single
person or team;
– Confirm those parts of a product in which improvement is
either not desired or not needed;
– Achieve technical work of more uniform, or at least more
predictable, quality than can be achieved without reviews,
in order to make technical work more manageable.
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Software Reviews
• A group examines part or all of a process or system and its
documentation to find potential problems.
• Software or documents may be 'signed off' at a
review which signifies that progress to the next
development stage has been approved by
management.
• There are different types of review with different objectives
– Inspections for defect removal (product);
– Reviews for progress assessment (product and process);
– Quality reviews (product and standards).
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Quality Reviews
• A group of people carefully examine part or all
of a software system and its associated documentation.
• Code, designs, specifications, test plans, standards, etc. can all
be reviewed.
• Software or documents may be 'signed off' at a review which
signifies that progress to the next development stage has
been approved by management.
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Program inspections
• These are peer reviews where engineers examine the source
of a system with the aim of discovering anomalies and
defects.
• Inspections do not require execution of a system so may be
used before implementation.
• They may be applied to any representation of the system
(requirements, design ,configuration data, test data, etc.).
• They have been shown to be an effective technique for
discovering program errors.
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Inspection checklists
• Checklist of common errors should be used to
drive the inspection.
• Error checklists are programming language
dependent and reflect the characteristic errors that are likely
to arise in the language.
• In general, the 'weaker' the type checking, the larger the
checklist.
• Examples: Initialisation, Constant naming, loop
termination, array bounds, etc.
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An inspection checklist (a)
Fault class Inspection check
Data faults  Are all program variables initialized before their values are used?
 Have all constants been named?
 Should the upper bound of arrays be equal to the size of the
array or Size -1?
 If character strings are used, is a delimiter explicitly assigned?
 Is there any possibility of buffer overflow?
Control faults  For each conditional statement, is the condition correct?
 Is each loop certain to terminate?
 Are compound statements correctly bracketed?
 In case statements, are all possible cases accounted for?
 If a break is required after each case in case statements, has it
been included?
Input/output faults  Are all input variables used?
 Are all output variables assigned a value before they are output?
 Can unexpected inputs cause corruption?
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An inspection checklist (b)
Fault class Inspection check
Interface faults  Do all function and method calls have the correct number
of parameters?
 Do formal and actual parameter types match?
 Are the parameters in the right order?
 If components access shared memory, do they have the
same model of the shared memory structure?
Storage management
faults
 If a linked structure is modified, have all links been
correctly reassigned?
 If dynamic storage is used, has space been allocated
correctly?
 Is space explicitly deallocated after it is no longer
required?
Exception management
faults
 Have all possible error conditions been taken into
account?
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Software Reliability
 Reliability is a measurable system attribute so non-functional
reliability requirements may be specified quantitatively. These
define the number of failures that are acceptable during
normal use of the system or the time in which the system
must be available.
• Functional reliability requirements define system and
software functions that avoid, detect or tolerate faults in the
software and so ensure that these faults do not lead to system
failure.
• Software reliability requirements may also be included to
cope with hardware failure or operator error.
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Software Reliability
• Reliability
– The probability of failure-free system operation over a
specified time in a given environment for a given purpose
• Availability
– The probability that a system, at a point in time, will be
operational and able to deliver the requested services
• Both of these attributes can be expressed quantitatively e.g.
availability of 0.999 means that the system is up and running
for 99.9% of the time.
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Reliability Specification Process
• Risk identification
– Identify the types of system failure that may lead to
economic losses.
• Risk analysis
– Estimate the costs and consequences of the different types
of software failure.
• Risk decomposition
– Identify the root causes of system failure.
• Risk reduction
– Generate reliability specifications, including quantitative
requirements defining the acceptable levels of failure.
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Types of system failure
Failure type Description
Loss of service The system is unavailable and cannot deliver its services to
users. You may separate this into loss of critical services and
loss of non-critical services, where the consequences of a
failure in non-critical services are less than the consequences of
critical service failure.
Incorrect service delivery The system does not deliver a service correctly to users. Again,
this may be specified in terms of minor and major errors or
errors in the delivery of critical and non-critical services.
System/data corruption The failure of the system causes damage to the system itself or
its data. This will usually but not necessarily be in conjunction
with other types of failures.
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Reliability Metrics
• Reliability metrics are units of measurement of system
reliability.
• System reliability is measured by counting the number of
operational failures and, where appropriate, relating these to
the demands made on the system and the time that the
system has been operational.
• A long-term measurement programme is required to assess
the reliability of critical systems.
• Metrics
– Probability of failure on demand
– Rate of occurrence of failures/Mean time to failure
– Availability
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Examples: Probability of failure on
demand (POFOD)
• This is the probability that the system will fail when a service
request is made. Useful when demands for service are
intermittent and relatively infrequent.
• Appropriate for protection systems where services are
demanded occasionally and where there are serious
consequence if the service is not delivered.
• Relevant for many safety-critical systems with exception
management components
– Emergency shutdown system in a chemical plant.
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Rate of fault occurrence (ROCOF)
• Reflects the rate of occurrence of failure in the system.
• ROCOF of 0.002 means 2 failures are likely in each 1000
operational time units e.g. 2 failures per 1000 hours of
operation.
• Relevant for systems where the system has to process a large
number of similar requests in a short time
– Credit card processing system, airline booking system.
• Reciprocal of ROCOF is Mean time to Failure (MTTF)
– Relevant for systems with long transactions i.e. where
system processing takes a long time (e.g. CAD systems).
MTTF should be longer than expected transaction length.
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Perceptions of reliability
• The formal definition of reliability does not always reflect the
user’s perception of a system’s reliability
– The assumptions that are made about the environment
where a system will be used may be incorrect
• Usage of a system in an office environment is likely to
be quite different from usage of the same system in a
university environment
– The consequences of system failures affects the perception
of reliability
• Unreliable windscreen wipers in a car may be irrelevant
in a dry climate
• Failures that have serious consequences (such as an
engine breakdown in a car) are given greater weight by
users than failures that are inconvenient
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Reliability and specifications
• Reliability can only be defined formally with respect to a
system specification i.e. a failure is a deviation from a
specification.
• However, many specifications are incomplete or incorrect –
hence, a system that conforms to its specification may ‘fail’
from the perspective of system users.
• Furthermore, users don’t read specifications so don’t know
how the system is supposed to behave.
• Therefore perceived reliability is more important in practice.
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Software quality assurance

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