ISO/IEC 25020:2007

INTERNATIONAL ISO/IEC
STANDARD 25020
First edition
2007-05-15


Software engineering — Software product
Quality Requirements and Evaluation
(SQuaRE) — Measurement reference
model and guide
Ingénierie du logiciel — Exigences de qualité du produit logiciel et
évaluation (SQuaRE) — Modèle de référence de mesure et guide




Reference number
ISO/IEC 25020:2007(E)
©
ISO/IEC 2007

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ISO/IEC 25020:2007(E)
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ISO/IEC 25020:2007(E)

Contents

Foreword . iv
Introduction.v
1 Scope .1
2 Conformance.1
3 Normative references .1
4 Terms and definitions.2
5 Symbols (and abbreviated terms).4
6 Software Product Quality Measurement .5
6.1 Software product quality measurement reference model (SPQM-RM) .5
6.2   Selecting software quality measures.5
6.3 Constructing software quality measures .6
Annex A (informative) Examples of criteria for selecting software quality measures and quality measure
elements.8
(informative)
Annex B Assessing measurement validity and reliability .10
(informative)
Annex C Example format for documenting software quality measures .12
Bibliography.15
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ISO/IEC 25020:2007(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission)
form the specialized system for worldwide standardization. National bodies that are members of ISO or IEC
participate in the development of International Standards through technical committees established by the
respective organization to deal with particular fields of technical activity. ISO and IEC technical committees
collaborate in fields of mutual interest. Other international organizations, governmental and non-governmental, in
liaison with ISO and IEC, also take part in the work. In the field of information technology, ISO and IEC have
established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International Standards
adopted by the joint technical committee are circulated to national bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this part of ISO/IEC 25020 may be the subject of
patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
ISO/IEC FDIS 25020 is part of the SQuaRE series of standards and was prepared by Joint Technical Committee
ISO/IEC JTC 1, Information technology, Subcommittee SC 7, Software and systems engineering.
SQuaRE series of standards consists of the following divisions under the general title Software engineering,
Software product Quality Requirements and Evaluation (SQuaRE):
Quality Management Division (ISO/IEC 2500n),
—
Quality Model Division (ISO/IEC 2501n),
—
— Quality Measurement Division (ISO/IEC 2502n),
Quality Requirements Division (ISO/IEC 2503n), and
—
Quality Evaluation Division (ISO/IEC 2504n).
—
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ISO/IEC 25020:2007(E)
Introduction
The general goal of creating the SQuaRE series of International Standards is to move to a logically organized,
enriched and unified series covering three complementary processes: requirements specification, measurement
and evaluation. The purpose of the SQuaRE series of International Standards is to assist those developing and
acquiring software products with the specification and evaluation of quality requirements. It establishes criteria for
the specification of software product quality requirements and their evaluation. It includes a two-part quality model
for aligning customer definitions of quality with characteristics of the software product. In addition, the series
defines measures of software product quality characteristics that can be used by developers, acquirers and
evaluators.

It has to be stressed that the SQuaRE series of International Standards is dedicated to software product quality
only. The Quality Management Division of the SQuaRE series deals with software products, and is separate and
distinct from the "Quality Management" of processes which is defined in the ISO 9000 family of International
Standards.

The major benefits of the SQuaRE series over its predecessor standards include:
— the coordination of guidance on software product quality measurement and evaluation,

— guidance for the specification of software product quality requirements, and

— harmonization with ISO/IEC 15939 in the form of Quality Measurement Reference model presented in this
International Standard.

The major differences between the SQuaRE series of International Standards and its predecessors, ISO/IEC 9126
and ISO/IEC 14598, are the:

— introduction of the new general reference model,

— introduction of dedicated and detailed guides for each division,

— introduction of Quality Measure Elements within the Quality Measurement Division,

— introduction of the Quality Requirements Division,

— incorporation and revision of the evaluation processes,

— introduction of guidance for practical use in the form of examples, and

— co-ordination and harmonization of content with ISO/IEC 15939.

SQuaRE consists of the following five divisions:

— Quality Management Division (ISO/IEC 2500n),

— Quality Model Division (ISO/IEC 2501n),

— Quality Measurement Division (ISO/IEC 2502n),

— Quality Requirements Division (ISO/IEC 2503n), and

— Quality Evaluation Division (ISO/IEC 2504n).

SQuaRE provides:

— terms and definitions,
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ISO/IEC 25020:2007(E)
— reference models,

— a general guide,

— individual division guides, and

— standards for requirements specification, measurement and evaluation purposes.

SQuaRE includes International Standards and technical reports for a quality model and measures, as well as on
quality requirements and evaluation. SQuaRE replaces current ISO/IEC 9126 series and ISO/IEC 14598 series.

This International Standard is intended to be used together with ISO/IEC 25010. It is strongly recommended that
users refer to ISO/IEC 2500n, ISO/IEC 2501n, ISO/IEC 2503n, and ISO/IEC 2504n division of International
Standards prior to using this International Standard and the associated measurement technical reports, particularly
if the user is not familiar with the use of software measures for requirements specification and product evaluation.
These International Standards discuss the planning and use of the software quality measures defined in the
ISO/IEC 2502n series on software product quality measurement.

ISO/IEC 25010 will provide a model and defines terms for software product quality characteristics and how these
characteristics are decomposed into subcharacteristics. It does not describe how any of these subcharacteristics
could be measured. The Quality Measurement Division provides information and guidance about how to measure
the characteristics and subcharacteristics of a quality model. This International Standard provides a reference
model and guide for measuring the quality characteristics defined in ISO/IEC 2501n Quality Model Division
(Figure 1). The associated standards and technical reports within the Quality Measurement Division describe
measures of quality throughout the product life cycle.

ISO/IEC 25021 offers quality measure elements that can be used to construct software quality measures. Quality
measure elements are the base and derived measures used to create measures of software product quality
characteristics. Quality measure elements may measure a static representation of the software, the behaviour of
the software, or the effects of the software when it is used.

ISO/IEC 25022, ISO/IEC 25023 and ISO/IEC 25024 will describe measures for the characteristics in the quality
model. Internal measures characterize software product quality based upon static representations of the software,
external measures characterize software product quality based upon the behaviour of the computer-based system
including the software, and quality in use measures characterize software product quality based upon the effects of
using the software in a specific context of use.

The measures in these technical reports should not be construed as an exhaustive or required set. Figure 2
depicts the relationship between this International Standard and the technical reports in the Quality Measurement
Division. Developers, evaluators, quality managers, acquirers, suppliers, maintainers and other users of software
may select measures from these technical reports for the measurement of quality characteristics of interest. In
practice this may be with respect to defining requirements, evaluating software products, quality management and
other purposes. Users may also modify the measures or use measures which are not included in those technical
reports.
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ISO/IEC 25020:2007(E)



Quality Model Division
2501n
Quality
Quality
Quality
Evaluation
Requirements
Management Division
Division
Division
2500n
2504n
2503n
Quality
Measurement Division
2502n


Figure 1 — Organization of the SQuaRE series of International Standards

2502502020
MeasMeasururemeemennt rt reeferfereencnce e
momoddeell a and nd gguuiiddee
2250250222
2502502323 2502502424
MMeaseasururememeent ont off MeasMeasururemeemennt ot off
MMeaseasururememeenntt o off q quuaallitityy
iinntteerrnnal qal quualalityity exexterternnalal q quualialittyy
inin u ussee
2502502121
QuaQuallityity me measasurure e
elelememeentsnts

Figure 2 — Structure of the Quality Measurement division
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INTERNATIONAL STANDARD ISO/IEC 25020:2007(E)

Software engineering — Software product Quality
Requirements and Evaluation (SQuaRE) — Measurement
reference model and guide
1 Scope
The scope of this International Standard is the selection and construction of software product quality measures,
with respect to their use in conjunction with the other SQuaRE series documents.
This International Standard also contains the following informative annexes (A through C) and Bibliography:
— Criteria for selecting software quality measures and quality measure elements
— Demonstrating predictive validity and assessing measurement reliability
— Example format for documenting software quality measures
— Bibliography
The SQuaRE series of International Standards is intended for, but not limited to, developers, acquirers and
independent evaluators of software, particularly those responsible for defining software product quality
requirements and for software product evaluation. It is recommended that users of SQuaRE use this International
Standard as a guide to execute their software product quality measurement tasks.
2 Conformance
Any software product quality measurement that conforms to this International Standard shall fulfil the requirements
of Clause 6.
3 Normative references
The following referenced documents are indispensable for the application of this document. For dated references,
only the edition cited applies. For undated references, the latest edition of the referenced document (including any
amendments) applies.
ISO/IEC 25000, Software Engineering — Software product Quality Requirements and Evaluation (SQuaRE) —
Guide to SQuaRE
ISO/IEC 15939:2002, Software engineering — Software measurement process


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ISO/IEC 25020:2007(E)

4 Terms and definitions
For the purposes of this standard, the terms and definitions given in ISO/IEC 25000 and ISO/IEC 15939 apply.
The following definitions are replicated here for the convenience of the user of this Internationl Standard.
Unattributed references are from ISO/IEC 25000.
4.1
attribute
inherent property or characteristic of an entity that can be distinguished quantitatively or qualitatively by human or
automated means

[ISO/IEC 15939:2002]
NOTE  ISO 9000 distinguishes two types of attributes: a permanent characteristic existing inherently in something; and an
assigned characteristic of a product, process or system (e.g. the price of a product, the owner of a product). The assigned
characteristic is not an inherent quality characteristic of that product, process or system.
4.2
base measure
measure defined in terms of an attribute and the method for quantifying it
NOTE  A base measure is functionally independent of other measures.
[ISO/IEC 15939: 2002, based on the definition in International Vocabulary of Basic and General Terms in
Metrology, 1993]
4.3
derived measure
measure that is defined as a function of two or more values of base measures
[ISO/IEC 15939:2002, based on the definition in International Vocabulary of Basic and General Terms in Metrology,
1993]
NOTE A transformation of a base measure using a mathematical function can also be considered as a derived measure.
4.4
external software quality
capability of a software product to enable the behaviour of a system to satisfy stated and implied needs when the
system is used under specified conditions
NOTE Attributes of the behaviour can be verified and/or validated by executing the software product during testing and
operation.
EXAMPLE The number of failures found during testing is an external software quality measure related to the number of faults
present in the program. The two measures are not necessarily identical since testing may not find all faults, and a fault may
give rise to apparently different failures in different circumstances.
4.5
indicator
measure that provides an estimate or evaluation of specified attributes derived from a model with respect to defined
information needs
[ISO/IEC 15939:2002]
NOTE In ISO/IEC 14598 this definition was, "a measure that can be used to estimate or predict another measure."
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ISO/IEC 25020:2007(E)


4.6
information need
insight necessary to manage objectives, goals, risks and problems
[ISO/IEC 15939:2002]
4.7
internal software quality
capability of a set of static attributes of a software product to satisfy stated and implied needs when the software
product is used under specified conditions
NOTE 1 Static attributes include those that relate to the software architecture, structure and its components.
NOTE 2 Static attributes can be verified by review, inspection and/or automated tools.
EXAMPLE The number of lines of code, complexity measures and the number of faults found in a walk through are all internal
software quality measures made on the product itself.
4.8
measure, noun
variable to which a value is assigned as the result of measurement
NOTE The term “measures” is used to refer collectively to base measures, derived measures and indicators.
[ISO/IEC 15939:2002]

4.9
measure, verb
make a measurement
[ISO/IEC 14598-1:1999]

4.10
measurement
set of operations having the object of determining a value of a measure
[ISO/IEC 15939:2002, based on the definition in International Vocabulary of Basic and General Terms in Metrology,
1993]
NOTE Measurement can include assigning a qualitative category such as the language of a source program
(ADA, C, COBOL, etc.).

4.11
measurement function
algorithm or calculation performed to combine two or more base measures
[ISO/IEC 15939:2002]
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ISO/IEC 25020:2007(E)


4.12
measurement method
logical sequence of operations, described generically, used in quantifying an attribute with respect to a specified
scale
[ISO/IEC 15939:2002, based on the definition in International Vocabulary of Basic and General Terms in Metrology,
1993]
4.13
quality in use (measure)
extent to which a product used by specific users meets their needs to achieve specific goals with
effectiveness, productivity, safety and satisfaction in specific contexts of use
4.14
quality measure element
measure, which is either a base measure or a derived measure, that is used for constructing software quality
measures
NOTE  The software quality characteristic or subcharacteristic of the entity is derived afterwards by calculating a software
quality measure.

5 Symbols (and abbreviated terms)
The following symbols and abbreviations are used in this International Standard.
SPQM-RM – Software Product Quality Measurement Reference Model
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ISO/IEC 25020:2007(E)

6 Software Product Quality Measurement
6.1 Software product quality measurement reference model
The software product quality measurement reference model (SPQM-RM) describes the relationship between a
quality model, its associated quality characteristics (and subcharacteristics), and software product attributes with
the corresponding software quality measures, measurement functions, quality measure elements, and
measurement methods. These relationships constitute the reference model for software product quality
measurement. Figures 3 and 4 depict the relationships between the quality model and the construction of quality
measures from quality measure elements. The measurement information model presented in Annex A of ISO/IEC
15939 describes the relationship between software product attributes and the measurement methods used to
produce base and derived measures which, in turn, may be used as quality measure elements. The left side of
Figure 3 shows that software product quality is composed of quality characteristics which in turn may be composed
of subcharacteristics. The decomposition of software product quality may go through several levels although the
nominal model contained in ISO/IEC 25010 only contains two levels. The right side of Figure 3 shows that software
quality measures are used to indicate the quality characteristics and subcharacteristics of interest. Refer to ISO/IEC
25030 for guidance on selecting quality characteristics and subcharacteristics of interest in conjunction with the
specification of quality requirements and ISO/IEC 25040 for guidance on using software quality measures for
software product evaluation.
Figure 3 also shows that software quality measures are constructed by applying quality measure elements to a
measurement function. In the special case where the quality measure element also serves as a software
quality measure, the measurement function applied would be the identity function.
Quality measure elements may be either base or derived measures. Quality measure elements are constructed in
accordance with the guidance provided in ISO/IEC 15939.

SoftSoftwwaarere Pr Prododuucctt

QuQuaallitityy
composecomposedd of of
Software
inindicatdicatee

QQuualalitityy M Meeaassurureses
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genergeneraatetess
ChChararacacteristeristticsics
MeMeasasurureemmeenntt

composecomposedd of of
FunctFunctiionon
indindicaticatee

arare ape appplieliedd ttoo
QuQuaallitityy

QuaQuallitityy M Measeasuurree
SuSubb-C-Chharaaracctteerriissttiiccss
ElemenElementsts

Figure 3 – Software Product Quality Measurement Reference Model (SPQM-RM)

6.2   Selecting software quality measures
Software quality measures are selected to satisfy the needs of developers, acquirers, managers, and others for
information. In the context of the SQuaRE series, information needs may be defined by quality requirements
specifications and product quality evaluations.
6.2.1 Criteria for selecting software quality measures and quality measure elements to fulfil those
information needs shall be documented.
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ISO/IEC 25020:2007(E)

At a minimum, the criteria shall be listed along with the assessment of the selected measures against the stated
criteria. Annex A provides examples of criteria for selecting software quality measures.
NOTE ISO/IEC 25030 and ISO/IEC 25040 provide guidance on quality requirements specifications and product quality
evaluations, respectively.
6.2.2 When using a modified or a new measure not identified in the ISO/IEC 25022, ISO/IEC 25023 or
ISO/IEC 25024, the user shall specify how the measure relates to its corresponding quality model
and how it is to be constructed from quality measure elements.
Annex C provides an example of how to document a quality measure.
NOTE ISO/IEC 25010 provides guidance on defining and using a software product quality model.
6.3 Constructing software quality measures
Three different types of software quality measures are defined to correspond to the software product quality life
cycle as shown in Figure 4. Internal software quality measures are applied to a part of a software product during its
development stages (such as request for proposal, requirements definition, design specification or source code).
Internal software product quality measurement provides users the ability to measure the quality of the intermediate
deliverables or work products. Additionally, these measurements may be used with an analysis model to predict
the quality of the final software product. This allows the user to detect software product quality issues and take
corrective actions during the early stages of the development life cycle process.
External software quality measures are used to measure the quality of the software product based on the behaviour
of the system of which it is a part. External software product quality measurement is used only during the testing
and operational stages of the product life cycle.
Measures of quality in use measure the extent to which a product meets the needs of specific users with respect to
their specific personal or business goals. These measures can only be made in a realistic and operational system
environment.
Throughout the product life cycle these software quality measures are made and used to assist with the successful
management of product development, evaluation and support (see Figure 4).
6.3.1. The definitions of the software quality measures used shall be documented.
The definitions shall include: the quality measure name, the corresponding quality characteristic and
subcharacteristic, the measurement focus (quality life cycle phase) it relates to, a purpose statement, the decision
criteria for interpretation and action, and identification of the quality measure elements used to construct it. It is
recommended, however, that the definition of the software quality measure contain all of the information included in
the example format provided in Annex C.
NOTE An example set of internal software quality measures along with their definitions is given in ISO/IEC 25022. An
example set of external software quality measures along with their definitions is given in ISO/IEC 25023. An example set of
quality in use software quality measures along with their definitions is given in ISO/IEC 25024.
6.3.2. The definitions of the quality measure elements needed to construct the software quality measures
shall be documented.
NOTE Quality measure elements are used throughout the software product lifecycle to construct measures of internal,
external and quality in use software product quality by applying measurement methods to specified attributes and, when
necessary, combining the resulting measures via a measurement function. The quality measure elements measure the
attributes of the software product itself, the effects of using the software product in a specific context and the resources
consumed or activities performed during software product development, testing, and maintenance. An example set of quality
measure elements along with their definitions is given in ISO/IEC 25021.

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ISO/IEC 25020:2007(E)

Needs
Requirements Product
Quality in
Quality in use
Use Specifying
Quality in use
Evaluating
Requirements
Measures
Validation
Determine Indicates
External
External Quality External
Quality
Evaluating
Specifying
Requirements Quality
Validation &
Measures
Verification
Determine Indicates
Internal
Internal Quality Internal
Quality
Specifying
Evaluating
Measures
Requirements Quality
Verification
Implementation

Figure 4 – Software product quality life cycle and software quality measures

NOTE Software quality measures are used to specify product requirements and to evaluate software product quality.

6.3.3 The user of the ISO/IEC 25020 international standards series shall plan and perform the
measurement in order to determine values of quality measure elements and software quality measures
following the reference model in Figure 3 and by using a procedure that conforms to ISO/IEC 15939.
NOTE  The user of the ISO/IEC 2502n division documents for software product quality measurement is encouraged to use
them in accordance with the product quality evaluation procedures contained in the ISO/IEC 2504n and the quality requirements
definition process contained in the ISO/IEC 2503n divisions of standards.

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ISO/IEC 25020:2007(E)

Annex A
  (informative)
Examples of criteria for selecting software quality measures and
quality measure elements
A.1 Criteria for selecting software quality measures and quality measure elements

Many different combinations of quality measure elements and software quality measures may be selected to
address a specific information need by the user of the ISO/IEC 2502n division of International Standards for
software product quality measurement. The following criteria are suggested for consideration when selecting
among alternatives:

• relevance to the prioritized informati
...