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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 27 Slide 1
Quality Management

Objectives
To introduce the quality management process and
key quality management activities
To explain the role of standards in quality
management
To explain the concept of a software metric,
predictor metrics and control metrics
To explain how measurement may be used in
assessing software quality and the limitations of
software measurement

Topics covered
Process and product quality
Quality assurance and standards
Quality planning
Quality control

Software quality management
Concerned with ensuring that the required
level of quality is achieved in a software
product.
Involves defining appropriate quality
standards and procedures and ensuring that
these are followed.
Should aim to develop a ‘quality culture’
where quality is seen as everyone’s
responsibility.

What is quality?
Quality, simplistically, means that a product should
meet its specification.
This is problematical for software systems
• There is a tension between customer quality
requirements (efficiency, reliability, etc.) and developer
quality requirements (maintainability, reusability, etc.);
• Some quality requirements are difficult to specify in an
unambiguous way;
• Software specifications are usually incomplete and often
inconsistent.

The quality compromise
We cannot wait for specifications to improve
before paying attention to quality
management.
We must put quality management
procedures into place to improve quality in
spite of imperfect specification.

Scope of quality management
Quality management is particularly important
for large, complex systems. The quality
documentation is a record of progress and
supports continuity of development as the
development team changes.
For smaller systems, quality management
needs less documentation and should focus
on establishing a quality culture.

Quality management activities
Quality assurance
• Establish organisational procedures and standards for
quality.
Quality planning
• Select applicable procedures and standards for a
particular project and modify these as required.
Quality control
• Ensure that procedures and standards are followed by
the software development team.
Quality management should be separate from
project management to ensure independence.

Quality management and software development
Software development
process
Quality management
process
D1 D2 D3 D4 D5
Standards and
procedures
Quality
plan
Quality review repor ts

The quality of a developed product is
influenced by the quality of the production
process.
This is important in software development as
some product quality attributes are hard to
assess.
However, there is a very complex and poorly
understood relationship between software
processes and product quality.
Process and product quality

Process-based quality
There is a straightforward link between process and
product in manufactured goods.
More complex for software because:
• The application of individual skills and experience is
particularly imporant in software development;
• External factors such as the novelty of an application or
the need for an accelerated development schedule may
impair product quality.
Care must be taken not to impose inappropriate
process standards - these could reduce rather than
improve the product quality.

Process-based quality
Define process
Develop
product
Assess pr oduct
quality
Standar dise
process
Improve
process
Quality
OK
No Yes

Define process standards such as how
reviews should be conducted, configuration
management, etc.
Monitor the development process to ensure
that standards are being followed.
Report on the process to project
management and software procurer.
Don’t use inappropriate practices simply
because standards have been established.
Practical process quality

Standards are the key to effective quality
management.
They may be international, national,
organizational or project standards.
Product standards define characteristics that
all components should exhibit e.g. a common
programming style.
Process standards define how the software
process should be enacted.
Quality assurance and standards

Encapsulation of best practice- avoids
repetition of past mistakes.
They are a framework for quality assurance
processes - they involve checking
compliance to standards.
They provide continuity - new staff can
understand the organisation by
understanding the standards that are used.
Importance of standards

Product and process standards

Problems with standards
They may not be seen as relevant and up-to-
date by software engineers.
They often involve too much bureaucratic
form filling.
If they are unsupported by software tools,
tedious manual work is often involved to
maintain the documentation associated with
the standards.

Involve practitioners in development. Engineers
should understand the rationale underlying a
standard.
Review standards and their usage regularly.
Standards can quickly become outdated and this
reduces their credibility amongst practitioners.
Detailed standards should have associated tool
support. Excessive clerical work is the most
significant complaint against standards.
Standards development

ISO 9000
An international set of standards for quality
management.
Applicable to a range of organisations from
manufacturing to service industries.
ISO 9001 applicable to organisations which
design, develop and maintain products.
ISO 9001 is a generic model of the quality
process that must be instantiated for each
organisation using the standard.

ISO 9001

ISO 9000 certification
Quality standards and procedures should be
documented in an organisational quality
manual.
An external body may certify that an
organisation’s quality manual conforms to
ISO 9000 standards.
Some customers require suppliers to be ISO
9000 certified although the need for flexibility
here is increasingly recognised.

ISO 9000 and quality management
Project 1
quality plan
Project 2
quality plan
Project 3
quality plan
Project quality
mana gement
Organisa tion
quality man ual
ISO 9000
quality models
Organisa tion
quality pr ocess
is used to de velop instantia ted as
instantia ted as
documents
Suppor ts

Documentation standards
Particularly important - documents are the tangible
manifestation of the software.
Documentation process standards
• Concerned with how documents should be developed,
validated and maintained.
Document standards
• Concerned with document contents, structure, and
appearance.
Document interchange standards
• Concerned with the compatibility of electronic documents.

Documentation process
Crea te
initial dr aft
Review
draft
Incorporate
review
comments
Re-draft
document
Proofread
text
Produce
final dr aft
Check
final dr aft
Layout
text
Review
layout
Produce
print masters
Print
copies
Stage 1:
Creation
Stage 2:
Polishing
Stage 3:
Production
Appr oved document
Appr oved document

Document standards
Document identification standards
• How documents are uniquely identified.
Document structure standards
• Standard structure for project documents.
Document presentation standards
• Define fonts and styles, use of logos, etc.
Document update standards
• Define how changes from previous versions are
reflected in a document.

Document interchange standards
Interchange standards allow electronic documents to
be exchanged, mailed, etc.
Documents are produced using different systems
and on different computers. Even when standard
tools are used, standards are needed to define
conventions for their use e.g. use of style sheets and
macros.
Need for archiving. The lifetime of word processing
systems may be much less than the lifetime of the
software being documented. An archiving standard
may be defined to ensure that the document can be
accessed in future.

Quality planning
A quality plan sets out the desired product
qualities and how these are assessed and
defines the most significant quality attributes.
The quality plan should define the quality
assessment process.
It should set out which organisational
standards should be applied and, where
necessary, define new standards to be used.

Quality plans
Quality plan structure
• Product introduction;
• Product plans;
• Process descriptions;
• Quality goals;
• Risks and risk management.
Quality plans should be short, succinct
documents
• If they are too long, no-one will read them.

Software quality attributes

Quality control
This involves checking the software
development process to ensure that
procedures and standards are being
followed.
There are two approaches to quality control
• Quality reviews;
• Automated software assessment and software
measurement.

Quality reviews
This is the principal method of validating the quality
of a process or of a product.
A group examines part or all of a process or system
and its documentation to find potential problems.
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).

Types of review
Review type Principal purpose
Design or program
inspections
To detect detailed errors in the requirements, design or code. A checklist of
possible errors should drive the review.
Progress reviews To provide information for management about the overall progress of the
project. This is b oth a process and a product review and is concerned with
costs, plans and schedules.
Quality reviews To carry out a t echnical analysis of product components or documentation to
find mismatches between the specification and the component design, code or
documentation and to ensure that defined quality standards have been followed.

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.
Quality reviews

Review functions
Quality function - they are part of the general
quality management process.
Project management function - they provide
information for project managers.
Training and communication function -
product knowledge is passed between
development team members.

Quality reviews
The objective is the discovery of system
defects and inconsistencies.
Any documents produced in the process may
be reviewed.
Review teams should be relatively small and
reviews should be fairly short.
Records should always be maintained of
quality reviews.

Comments made during the review should be
classified
• No action. No change to the software or documentation is
required;
• Refer for repair. Designer or programmer should correct
an identified fault;
• Reconsider overall design. The problem identified in the
review impacts other parts of the design. Some overall
judgement must be made about the most cost-effective
way of solving the problem;
Requirements and specification errors may
have to be referred to the client.
Review results

Software measurement and metrics
Software measurement is concerned with deriving a
numeric value for an attribute of a software product
or process.
This allows for objective comparisons between
techniques and processes.
Although some companies have introduced
measurement programmes, most organisations still
don’t make systematic use of software
measurement.
There are few established standards in this area.

Any type of measurement which relates to a
software system, process or related documentation
• Lines of code in a program, the Fog index, number of
person-days required to develop a component.
Allow the software and the software process to
be quantified.
May be used to predict product attributes or to
control the software process.
Product metrics can be used for general predictions
or to identify anomalous components.
Software metric

Predictor and control metrics
Mana gement
decisions
Contr ol
measur ements
Softw are
process
Predictor
measur ements
Softw are
product

A software property can be measured.
The relationship exists between what we can
measure and what we want to know. We can only
measure internal attributes but are often more
interested in external software attributes.
This relationship has been formalised and
validated.
It may be difficult to relate what can be measured to
desirable external quality attributes.
Metrics assumptions

Internal and external attributes
Relia bility
Number of procedur e
par ameters
Cyclomatic comple xity
Program siz e in lines
of code
Number of error
messa ges
Length of user man ual
Maintaina bility
Usability
Por tability

The measurement process
A software measurement process may be
part of a quality control process.
Data collected during this process should be
maintained as an organisational resource.
Once a measurement database has been
established, comparisons across projects
become possible.

Product measurement process
Measur e
component
characteristics
Identify
anomalous
measur ements
Analyse
anomalous
components
Select
components to
be assessed
Choose
measur ements
to be made

Data collection
A metrics programme should be based on a
set of product and process data.
Data should be collected immediately (not in
retrospect) and, if possible, automatically.
Three types of automatic data collection
• Static product analysis;
• Dynamic product analysis;
• Process data collation.

Data accuracy
Don’t collect unnecessary data
• The questions to be answered should be
decided in advance and the required data
identified.
Tell people why the data is being collected.
• It should not be part of personnel evaluation.
Don’t rely on memory
• Collect data when it is generated not after a
project has finished.

A quality metric should be a predictor of
product quality.
Classes of product metric
• Dynamic metrics which are collected by measurements
made of a program in execution;
• Static metrics which are collected by measurements
made of the system representations;
• Dynamic metrics help assess efficiency and reliability;
static metrics help assess complexity, understandability
and maintainability.
Product metrics

Dynamic and static metrics
Dynamic metrics are closely related to software
quality attributes
• It is relatively easy to measure the response time of a
system (performance attribute) or the number of failures
(reliability attribute).
Static metrics have an indirect relationship with
quality attributes
• You need to try and derive a relationship between these
metrics and properties such as complexity,
understandability and maintainability.

Software product metrics
Software metric Description
Fan in/Fan-out Fan-in is a measure of the number of functions or methods that call some other function
or method (say X). Fan-out is the number of functions that are called by function X. A
high value for fan-in means that X i s tightly coupled to the rest of the design and
changes to X will have extensive knock-on effects. A high value for fan-out suggests
that the overall complexity of X may be high because of the complexity of the control
logic needed to coordinate the called components.
Length of code This is a measure of the size of a program. Generally, the larger the size of the code of a
component, the more complex and error-prone that component is likely to be. Length of
code has been shown to be one of the most reliable metrics for predicting error-
proneness in components.
Cyclomatic complexity This is a measure of the control complexity of a program. This control complexity may
be related to program understandability. I discuss how to compute cyclomatic
complexity in Chapter 22.
Length of identifiers This is a measure of the average length of distinct identifiers in a program. The longer
the identifiers, the more likely they are to be m eaningful and hence the more
understandable the program.
Depth of conditional
nesting
This is a measure of the depth of nesting of if-statements in a program. Deeply nested if
statements are hard to understand and are potentially error-prone.
Fog index This is a measure of the average length of words and sentences in documents. The higher
the value for the Fog index, the more difficult the document is to understand.

Object-oriented metrics
Object-oriented
metric
Description
Depth of inheritance
tree
This represents the number of discrete levels in the inheritance tree where sub-
classes inherit attributes and operations (methods) from super-classes. The
deeper the inheritance tree, the more complex the design. Many different object
classes may have to be understood to understand the object classes at the leaves
of the tree.
Method fan-in/fan-
out
This is directly related to fan-in and fan-out as described above and means
essentially the same thing. However, it may be appropriate to make a
distinction between calls from other methods within the object and calls from
external methods.
Weighted methods
per class
This is the number of methods that are included in a class weighted by the
complexity of each method. Therefore, a simple method may have a complexity
of 1 and a large and complex method a much higher value. The larger the value
for this metric, the more complex the object class. Complex objects are more
likely to be more difficult to understand. They may not be logically cohesive so
cannot be reused effectively as super-classes in an inheritance tree.
Number of
overriding
operations
This is the number of operations in a super-class that are over-ridden in a sub-
class. A high value for this metric indicates that the super-class used may not be
an appropriate parent for the sub-class.

Measurement analysis
It is not always obvious what data means
• Analysing collected data is very difficult.
Professional statisticians should be
consulted if available.
Data analysis must take local circumstances
into account.

Measurement surprises
Reducing the number of faults in a program
leads to an increased number of help desk
calls
• The program is now thought of as more reliable
and so has a wider more diverse market. The
percentage of users who call the help desk may
have decreased but the total may increase;
• A more reliable system is used in a different
way from a system where users work around
the faults. This leads to more help desk calls.

Key points
Software quality management is concerned
with ensuring that software meets its
required standards.
Quality assurance procedures should be
documented in an organisational quality
manual.
Software standards are an encapsulation of
best practice.
Reviews are the most widely used approach
for assessing software quality.

Key points
Software measurement gathers information
about both the software process and the
software product.
Product quality metrics should be used to
identify potentially problematical
components.
There are no standardised and universally
applicable software metrics.

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