Generic RE Quality Models

Over the years, various researchers have approached the concept of RE quality.

Along the most prominent line of research, Pohl [Poh93] models RE quality along three fundamental dimensions (see Fig. 2.9). These dimensions are specification (degree of completeness),representation (degree of formalization), andagreement (degree to which a common view was obtained). In the understanding of Pohl, the RE process is then a path within this model, towards a complete, formal and commonly agreed specification.

2.4. Related Work on Quality Models in Requirements Engineering

Around the same time, Lindland, Sindre and Sølvberg [LSS94] were also discussing the shortcomings of existing quality definition³. In particular, in RE at the time, more and more researchers tried to apply various forms of modelling. To deal with this understanding of RE as conceptual models, and for a more systematic approach towards quality than a list of characteristics, Lindland et al. developed a novel framework for the quality of conceptual models. The framework is based on the following concepts:

Model are all statements made by the conceptual model.

Language are all statements possible with the vocabulary and grammar provided.

Domain are all statements that are “correct and relevant about the problem at hand.”

([KLS95])

Audience interpretation are all statements which the audience perceives to be con-tained in the model.

Model Domain

Audience Interpretation

Language appropriateness

appropriateness appropriateness

pragmatic quality semantic

quality syntactic

quality

Figure 2.10.:This figure shows the terminology of the semiotic quality model by Lindland et al. as defined in [LSS94]. Figure taken from [KLS95].

Based on these basic concepts, the framework defines appropriateness, as well as three types of quality (see Fig. 2.10):

Appropriateness is degree of fitness between domain, language and audience.

Syntactic quality is the extent to which the model is correct with respect to the applied language.

Semantic quality is the extent to which the model is correct with respect to the domain.

Pragmatic quality is the extent to which the model is understood by the audience.

Afterwards, the framework classifies existing quality characteristics into these cate-gories.

In comparison, despite different terminology, both models address similar issues. As Krogstie, Lindland, and Sindre [KLS95] analyze, Pohl’s model aims for complete formalization, which might not necessarily be “always desirable” ([KLS95]). How-ever, Lindland’s model misses the characteristicagreement. Therefore, Krogstie et al. [KLS95] include the missing concepts proposed by Pohl into the Lindland model through the following concepts:

3 The existing quality definitions at that time were lists of characteristics, very similar to what is still considered best practice in standards.

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2. Fundamentals and Related Work

Participant Knowledge is understood as the union of the knowledge of the problem of all actors in the audience (i.e. “all possible statements that would be correct and relevant for addressing the problem at hand according to the knowledge of the actor” ([KLS95]).

Perceived semantic quality is analogue to semantic quality: Whereas semantic quality is the correctness of the model with respect to the domain, the perceived semantic quality, is the correctness of the interpretation of the model with respect to the understanding of the domain.

Social quality is defined as the agreement between social actors.

Finally, Krogstie [Kro98] furthermore extends the framework by categorizing the proposed quality characteristics of Davis et al. [DOJ⁺93] into the framework and adding a last quality facet:

Physical quality consists of externalization and internalization. Externalization means that the knowledge of all actors can be contained in the model. Internalization means that the model is persisted and available to the actors.

Besides this, Krogstie added various characteristics of appropriateness, which we leave out for the sake of clarity.

2.4.2. Specific RE Quality Models

In addition to these generic models, various authors have introduced specific quality models, depending on the representation used. For example, Berry et al. [BBG⁺06]

define a quality model for natural-language specifications combining their previous works [FFGL00, BKK03, BK04] in this area. For this, they define a set of quality characteristics, similar to the ones discussed in requirements standards, as well as a set of manifestations of problems, i.e. lexical, syntactic, structural and semantic.

Afterwards they classify various defects in natural language according to the model.

Similar approaches exist for use case quality [AS02, AS02, FGLM02] as well as user story quality [Coh04, LDBvdW15, LDvdWB16]. All these models have in common to focus on intrinsic properties of artifacts rather than on its usage for the engineering endeavor.

2.4.3. RE Quality Models in Standards

To explain the status quo of RE quality in standards, in the following we refer to two of the current standards. First, according to a recent systematic analysis by Schneider and Berenbach [SB13]), the ISO-29148 [ISO11b] “is actually the standard that every requirements engineer should be familiar with” ([SB13]). Second, the International Requirements Engineering Board (IREB) recently received more and more attention through their certification. Therefore, in the following, we discuss the perspectives on quality of these two institutions.

2.4.3.1. ISO/IEEE/IEC-29148

The ISO-29148 [ISO11b] is the most current standard applicable to RE. As such, it replaced the now superseded IEEE Recommended Practice for Software Requirements Specifications (IEEE-830) [IEE98]. According to Schneider and Berenbach, it can be considered “the mother of all requirements standards as it gives a rather extensive description of the domain of requirements engineering” ([SB13]). It is tailored particularly to systems engineering, but can be applied to various types of software-intensive systems in various domains.

The standard specifies:

2.4. Related Work on Quality Models in Requirements Engineering

Set of Reqs. / Reqs. Document

Complete Affordable

Bounded Consistent

Unambiguity Clear Structure Modifiability and

Extensibility Traceability

Requirements Language Criteria

Subjective Language Vague Pronouns

Superlatives

Ambiguous Adverbs and Adjectives Open-ended,

non-verifiable. Terms Comparatives

Negatives Statements Loopholes Incomplete References

Short Sentences and Paragraphs

One Req. per Sentence (Individual)

Requirements

Necessary

Implementation Free Unambiguous

Consistent Complete

Singular Feasible Traceable Verifiable

Agreed Understandable

ISO 29148 Characteristic ISO 29148 & IREB

Characteristics

IREB Characteristics Key:

Figure 2.11.:This figure depicts the quality characteristics of ISO 29148 and the IREB syllabus in comparison. Blue characteristics are shared characteristics, orange and green characteristics appear only in one of the standards. Please note that, as we discuss in the text, some characteristics are shared between the standards by their name, but vary in the precise meaning of the characteristics.

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2. Fundamentals and Related Work

• Definitions for the most important concepts in RE

• Processes for software and systems requirements

• Required information items to be produced during these processes

• Content of these information items

• Guidelines for information items through templates and rules for natural language

• Relations to other software lifecycle standards

For this thesis, we are especially interested in the understanding of quality, according to the standard.

RE artifact quality according to ISO-29148. According to the ISO-29148 standard, a good RE artifact is one that follows its templates and formats. Regarding the content, the standard defines a set of quality characteristics, which we depicted in blue and orange in Fig. 2.11. The ISO 29148 defines the characteristics on three levels: In the level of a set of requirements, individual requirements, and requirements language.

At the highest level, according to ISO 29148, a set of requirements should be [ISO11b, pp.11-12]:

• Consistent: The ISO 29148 subsumes three aspects under this category.

No individual requirements is contradictory, no individual requirements is duplicated, and the same term is used for the same concept throughout the whole set of requirements.

• Complete: The ISO 29148 understand as complete that “the set of require-ments needs no further amplification because it contains everything pertinent to the definition of the system or system element being specified.” ([ISO11b, p.11]) In addition, the set does not contain any TBx’s, such asto be defined (TBD).

• Affordable: The ISO 29148 requires that the “set of requirements can be satisfied by a solution that is obtainable/feasible within life cycle constraints”

([ISO11b, p.12]).

• Bounded: Lastly, ISO 29148 requires that the specified requirements stay within the identified scope.

At the second level, ISO 29148 defines nine characteristics for each individual requirement [ISO11b, p.11]:

• Unambiguous: This combines two aspects: First, requirements can be only interpreted in one way, and, second, requirements are easy to understand.

• Necessary: This combines the following aspects: First, a removal of the requirement leads to a deficiency. Second, the requirement is not obsolete and, last, planned expiration is clearly identified.

• Consistent: An individual requirement is consistent if it is free of conflicts.

• Complete: An individual requirement is complete if it needs no further amplification. In particular, “it is measurable, and sufficiently describes the capability and characteristics to meet the stakeholder’s need.” ([ISO11b, p.11])

• Traceable: An individual requirement is traceable if it is both upward-traceable to its source, and downward-upward-traceable to derived requirements or implementation artifacts.

• Verifiable:An individual requirements is verifiable if, based on the requirement, it is possible to prove that the system satisfies the requirement. The standard furthermore states that “Evidence may be collected that proves that the system can satisfy the specified requirement.” ([ISO11b, p.11])

2.4. Related Work on Quality Models in Requirements Engineering

• Feasible: An individual requirement is feasible if it is technically achiev-able without major technology advances and within system constraints and acceptable risks.

• Implementation Free: An individual requirement is implementation free if it does not add unnecessary constraints to the design.

• Singular: An requirements statement⁴ is singular if it only contains one requirement. In particular, it is not singular if it contains conjunctions.

Lastly, at the lowest level, the standard defines a set of requirements language criteria⁵. This set defines various grammatical and lexical critera which should be avoided. In particular, the list contains comparatives, superlatives andvague pronouns as grammatical features to avoid, and subjective language, ambiguous adverbs and adjectives, loopholes, andnegative statements as lexical phrases to avoid.

Lastly, it mentions incomplete references.

Issues in ISO-29148. In addition to the general critique motivated in Chapter 1.1, the details of the standard reveal a broader list of issues in the details of the ISO 29148’s quality definition. In the following, we briefly list a subset of the problems:

• The characteristictraceable is defined in a recursive manner.

• The characteristic verifiability requires that a requirement is provable. Assum-ing a proof to be a formal verification, we argue that for most requirements this definition is hardly achievable in practice. Hence, the definition adds that

“evidence may be collected that proves that the system can satisfy the (...) requirement.” ([ISO11b, p.11]) However, this latter definition is a significant weakening of the former. The ISO 29148 leaves it to users of the standard to interpret which definition to use and how to interpret it.

• However, the list requirements language criteria is only part of the recom-mendations for natural language. Throughout the document, the standard provides further implicit or explicit suggestions for higher quality, such as the usage of active voice [ISO11b, p.10].

• The characteristicconsistencyis not defined consistently. While for an individ-ual requirement, it just refers to freedom of conflicts, for a set of requirements, it also refers to duplication and consistent usage of terminology.

• One could discuss to which extentnecessary andimplementation free refer to the same criterion. Every violation of implementation free, is also a violation of necessity. The same holds for unambiguity and verifiability. Every ambiguous requirement is by definition not verifiable.

2.4.3.2. IREB

The IREB⁶ is a nonprofit organization, which was founded in 2006, and which is the provider of the CPRE (Certified Professional for Requirements Engineering) certification scheme. As such, IREB is not a standardization authority such as ISO, IEEE or others. Instead, it provides certifications for requirements engineers in practice. All in all, more and more companies make use of this certification authority.

IREB announced that, over the last years, over 27,000 practitioners in 66 countries have passed their examination⁷.

4 Please note that in contrast to the remaining characteristics, ISO 29148 defines singularity based on a requirement statement, instead of a requirement.

5 Please note that, in the following, we will use the termcharacteristics on the abstract quality definition level, andcriteria for concrete (e.g. language) guidelines.

6http://www.ireb.org

7https://www.ireb.org/service/statistics/

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2. Fundamentals and Related Work Therefore, IREB does not standardize RE or RE artifacts. However, due to their widespread acceptance, the course material of IREB serves as a standard commonly accepted in industry. The following discussion of IREB’s understanding of quality builds upon two sources: The definitions, provided by Glinz [Gli14], and the current syllabus for the certification [Int15]. In the following, we assume that the official IREB syllabus is based on the official IREB glossary provided by Glinz⁸.

RE artifact quality according to IREB. In the IREB Glossary, quality is described in an abstract manner, instead of a concrete quality model: “Quality: The degree to which a set of inherent characteristics of an entity fulfills requirements.” ([Gli14, p.16]) Therefore, in order to understand RE artifact quality, IREB asks us to define the requirements to an RE artifact.

In the syllabus [Int15], this is broken down into concrete characteristics⁹. Again the characteristics are differentiated through three levels: the requirements document level, the requirements level, and some rules for natural language [Int15, p.16]. In the following, we describe these characteristics, in relation to the ISO 29148 (see Fig. 2.11).

For the requirements document level, IREB describes 6 characteristics, 2 of which are shared with the ISO 29148 (i.e. consistency and completeness). IREB adds unambiguity, clear structure, modifiability and extensibility, andtraceability.

• Consistency: Just as the 29148, IREB defines consistency as being free of contradicting statements [Gli14, p.11]. However, this does not include redundancy or specific references to terminology.

• Completeness: Completeness is defined as the degree to which all information is present that is required for developing the correct system [Gli14, p.10]. As such, the definition of completeness in IREB is very similar to the definition in 29148.

• Unambiguity: Glinz defines unambiguity as the degree to which a require-ment cannot be understood in multiple ways. It is not explicated in either document, but we assume this is the same definition applied also for a require-ments document.

• Clear Structure: This term remains undefined in [Gli14] and [Int15].

• Modifiability and Extensibility: These terms remain undefined in [Gli14]

and [Int15]. However,changeabilityis defined as the degree to which an artifact can be modified [Gli14, p.10].

• Traceability: Glinz only defines traceability at the level of individual require-ments. It remains undefined for a requirements document.

For each requirement, IREB defines the following 9 characteristics. It contains 7 characteristics shared with 29148 (namely unambiguous, necessary, consistent, complete, traceable, verifiable, feasible, and adds two (agreed andunderstandable; see Fig. 2.11).

• Unambiguous: (see above).

• Necessary: Remains undefined.

8 Please note that as required by ISO/IEC 17024:2012 [ISO12], IREB is not involved in the certification process. Hence, IREB provides neither the training, nor the certification themselves.

As such, in contrast to the syllabus, the training material can vary between training facilities.

Therefore, we refrained from using additional resources such as mandatory training material, since these are not obligatory for practitioners taking the training.

9 IREB refers to these characteristics asquality criteria. To maintain consistency, we will use the termcharacteristics.

2.4. Related Work on Quality Models in Requirements Engineering

• Consistent: Consistency is only defined at the level of a set of requirements, see above.

• Complete: Glinz defines completeness for a single requirement as the degree to which the requirement contains all necessary information [Gli14, p.10].

• Traceable: According to Glinz, traceability relates requirements with its origins, with its implementation, and with requirements it depends on [Gli14, p.22].

• Verifiable: Glinz defines verifiability as “the degree to which the fullfillment of a requirement (...) can be checked” ([Gli14, p.23]).

• Feasible: Remains undefined.

• Agreed: Remains undefined.

• Understandable: Remains undefined.

For natural language, the IREB syllabus provides few guidance. At the respective location, it advices only short sentences and singularity. One could argue that the transformational processes of language effects [Int15, p.18] could further be interpreted as such language guidance. For the rest, the syllabus promotes sentence patterns.

All in all, the IREB’s understanding of quality remains unsatisfying. As mentioned above, the syllabus just names the characteristics and does not provide definitions.

While the terms that are defined in the glossary provide rather clear and sensible definitions (except maybe for the recursive definition of traceability), many terms are not defined at all. Therefore, we argue that there is a need for more consistency between glossary and syllabus.

2.4.3.3. Discussion: Comparison between the ISO and IREB Standard

The IREB and ISO 29148 views on quality agree on some parts, but differ all in all.

We first discuss the agreements, before the differences. Afterwards, we describe our conclusions from the discussion.

The standards both define a quality model through a simple list of characteristics.

According to the standards, good requirements documents are those in which the characteristics are fulfilled. The standards share nine of the characteristics (see Fig. 2.11), mostly those characteristics that were defined in earlier literature and standards, such as the IEEE 830 [IEE98].

However, the standards disagree on more characteristics than they agree on. In particular, the standards completely disagree when it comes to concrete language criteria. And even when the standards agree on the characteristics, as soon as they define the characteristics, their interpretations differs significantly. Take, for example, consistency. While the IREB takes only disagreeing requirements into account, the 29148 also understands issues with duplication and terminology under this definition.

In addition, the definitions of Glinz take the characteristics as continuous variables, wheres the definitions of the standard are binary.

In summary, while the two standards take the same approach towards quality, as soon as they get more concrete, they differ tremendously. This is especially true for the concrete, assessable language criteria. We argue that these differences indicate two problems. First, the missing agreement at the level of concrete language criteria indicates that we do not yet know what is good or bad quality, and that we have little to no established understanding of the impacts of concrete language criteria.

Second and even more problematic, the missing agreement at the level of abstract quality characteristics indicates there is no established understanding and approach towards quality for RE artifacts as a whole. Although Glinz’ [Gli14] proposed

28

2. Fundamentals and Related Work definition of quality could be such an approach, the concrete definitions in the syllabus unfortunately does not follow this definition.

2.5. Research Gap

In the following, we summarize the research gaps. We look at quality definitions first, before discussing QA afterwards.

2.5.1. Research Gap: Quality Definitions for RE artifacts

To summarize this section, there are various viewpoints onto quality, namely:

Generic theories: Various authors have worked on approaches to define RE quality in a systematic way. While these approaches are helpful to define quality factors, their generic approach does not take into account that requirements engineering is just a means and not an end. For one example, Pohl [Poh93]

assumes that formalization is a goal of the RE process. We disagree with this view, since a formal model that is never used nor understood has no purpose in a software engineering project.

Standards and generic quality characteristics: Unfortunately, the current standards for RE quality are rather a list of characteristics than a structured approach towards quality. Although IREB contains some hints towards thinking about RE artifact usage, this is not yet reflected in the quality model. In particular, the strong differences between the currently most relevant standards show how arbitrary the existing characteristics are, and that we do not have a systematic

Standards and generic quality characteristics: Unfortunately, the current standards for RE quality are rather a list of characteristics than a structured approach towards quality. Although IREB contains some hints towards thinking about RE artifact usage, this is not yet reflected in the quality model. In particular, the strong differences between the currently most relevant standards show how arbitrary the existing characteristics are, and that we do not have a systematic

Im Dokument Requirements Engineering Artifact Quality: Definition and Control  (Seite 33-0)