The document discusses the importance of both functional and non-functional requirements (NFRs) in software development, using the metaphor of Snow White and her dwarfs to illustrate how NFRs are essential to the integrity of an application. It highlights the common pitfalls in not allocating sufficient attention to NFRs, which can lead to technical debt and maintenance challenges, urging teams to consider them during the development process. Additionally, it touches on quality models that can aid in the classification and assessment of these requirements to improve software quality.

1. Requirements
Engineering:
Quality Models
CECS 542
Dr. Birgit Penzenstadler 1
Photo credit: Ma7hew Kane, Unsplash

2. Mo=va=on
Dr. Birgit Penzenstadler 2
[Gonçalo Borrêga: The truth about non-func=onal requirements. Blog entry, 2013.
h7p://www.outsystems.com/blog/2013/03/the-truth-about-non-func=onal-requirements-nfrs.html]

3. Func=onal Requirements and Their
Poor Cousins: The Truth About NFRs
Dr. Birgit Penzenstadler 3
• Whenever anybody says Func=onal Requirement, I think of
princesses. I think of Arielle and Cinderella. I think of how
each is central to her story and embodies a specific
iden=ty, and then I think of the princess who stands out as
a true metaphor for func=onal requirements – the one
who reflects the role perfectly. I think of Snow White.
• Snow White is a func=onal requirement if I ever saw one.
She is at once central to her story, its main protagonist, its
raison d’etre, yet surrounded by a host of suppor=ng
characters – dwarfs – whose roles are necessary for
the story to be complete. No dwarfs, no real story. It’s
that simple. If a single dwarf is missing, the en=re story is
compromised. Snow White is compromised.
[Gonçalo Borrêga: The truth about non-func=onal requirements. Blog entry, 2013.
h7p://www.outsystems.com/blog/2013/03/the-truth-about-non-func=onal-requirements-nfrs.html]

4. Dr. Birgit Penzenstadler 4
• And it’s nearly the same with applica=ons. If an applica=on is the
story and Snow White is the func=onal requirement, then we can think of
dwarfs as non-func=onal requirements (NFRs). If a single NFR is missing
from an applica=on then that applica=on is compromised. Deploying a
squirrely applica=on is the same as adding maintenance issues and
technical debt directly to your applica=on por`olio. Eventually, that
applica=on will have to be addressed, those NFRs will have to be added,
and the IT department who deployed the app will have to contend with
the cost of changing sobware.
• How does this happen? Changing and maintaining sobware is hard
and therefore expensive to do and IT departments are oben rushed
or underfunded. However, if they are in a ‘Just do it’ mode then
those opera=onal, non-func=onal requirements that make an applica=on
complete are easy to leave out or are ‘forgo7en.’ The consequences
of leaving these NFRs out of an applica=on lead directly to the
aforemen=oned maintenance problems and increased technical debt.
• Here’s the list of most common NFRs that we see our customers worrying
about:
Func=onal Requirements and Their
Poor Cousins: The Truth About NFRs
[Gonçalo Borrêga: The truth about non-func=onal requirements. Blog entry, 2013.
h7p://www.outsystems.com/blog/2013/03/the-truth-about-non-func=onal-requirements-nfrs.html]

5. Dr. Birgit Penzenstadler 5
Common NFRs
[Gonçalo Borrêga: The truth about non-func=onal requirements. Blog entry, 2013.
h7p://www.outsystems.com/blog/2013/03/the-truth-about-non-func=onal-requirements-nfrs.html]

6. Dr. Birgit Penzenstadler 6
Common NFRs
[Gonçalo Borrêga]

7. Dr. Birgit Penzenstadler 7
Common NFRs
[Gonçalo Borrêga]

8. Dr. Birgit Penzenstadler 8
Common NFRs
[Gonçalo Borrêga]

9. Dr. Birgit Penzenstadler 9
Common NFRs
[Gonçalo Borrêga]

10. Dr. Birgit Penzenstadler 10
Func=onal Requirements and Their
Poor Cousins: The Truth About NFRs
• Like Snow White’s dwarfs these NFRs are necessary to comple=ng the story of the
applica=on. While you might consider 2 or 3 important NFRs for a project (like
performance and security), you’ll probably not cover the others extensively
enough, or you might miss out on them all together.
• And if you do allocate =me to deal with them all, when the project schedule slips,
the NFRs may be the first thing you’ll drop… because no one really sees them, and
your team will be looking at the func=onal requirements instead.
• So, whether you plan for NFRs or not, chances are high you won’t cover
them 100% of the =me in your development project. You’ll compromise and not
think of the whole story – the whole applica=on.
• But you should try. You should try to avoid adding technical debt and maintenance
nightmares to your future por`olio whenever possible. The cost of change is real,
and the moment you deploy your app, you’ll have to address its problems.
• From your experience what are the NFRs you constantly see developers
and project teams dropping most oben?

11. Quality models and dealing with NFRs
• Usage of Quality models in RE
• Exemplary quality models
• Dealing with NFRs in AMDiRE
Dr. Birgit Penzenstadler 11
K Rayker, stock.xchng

12. Usage of quality models in RE
12
What are quality models?
à Conceptual models for the descrip=on of quality.
Usage of quality models in RE
• Defini=on and assessment of sobware quality –
beginning in RE
• Quality assurance, for example of artefacts in RE
• Also: Classifica=on of requirements according to
characteris=cs

13. Usage of quality models in RE
13
Classifica=on on the basis of quality models
• Classifica=on of non-func=on requirements according to
characteris=cs.
– Which different classes of requirements exist?
– Which aspects are important to consider?
– Which modeling concepts and interdependencies are important to
consider?
Delimita=on: Artefact models
• Ideally build on quality models
(compare to system models)
à Concept model (Content Model)
à Structure of the concept model
(Structure Model)
Artefact-based RE Approach for Business Information Systems (Basic Components)
Structure
Model
Artefact Model
Content
Model
Role Model
Process Model
Tool SupportCustomisation
Approach
MetaModelREReferenceModel
Structure Content
Project-specific
Exemplars
instanceofinstanceof
!
PRODUKT.PROJEKTBEZEICHNUNG - PRODUKT.NAME
Zuletzt geändert: 27.10.2010 13:28 3/20
Content
1! Introduction.......................................................................................................................... 6!
1.1! Overview....................................................................................................................... 6!
1.2! Purpose.......................................................................................................................... 6!
1.3! References..................................................................................................................... 7!
1.4! Scope............................................................................................................................. 8!
2! System Vision ...................................................................................................................... 8!
2.1! Summary of Business Specification.............................................................................. 8!
2.2! Scope of Information System under Consideration ...................................................... 8!
2.2.1! System Overview ................................................................................................... 8!
2.2.2! External Systems.................................................................................................. 10!
2.2.3! Use Case Overview.............................................................................................. 10!
2.2.4! Information System Service Overview ................................................................ 10!
3! Information System Requirements..................................................................................... 11!
3.1! Actors .......................................................................................................................... 11!
3.2! Generic Scenarios........................................................................................................ 11!
3.3! Domain-specific Application Capabilities .................................................................. 12!
3.3.1! <<Business Domain>> <Name>.......................................................................... 12!
3.4! Information System Objects........................................................................................ 14!
3.5! System Quality Requirements..................................................................................... 16!
3.6! Architectural Constraints............................................................................................. 16!
3.6.1! Logical Restrictions.............................................................................................. 17!
3.6.2! Technical Restrictions.......................................................................................... 17!
4! Integrational Requirements ................................................................................................ 18!
4.1! Deployment Requirements.......................................................................................... 18!
4.2! Migration Requirements.............................................................................................. 18!
5! Organisational Requirements............................................................................................. 19!
5.1! Project Requirements .................................................................................................. 19!
5.2! Obligations .................................................................................................................. 19!
5.3! Glossary....................................................................................................................... 19!
6! Abbreviations ..................................................................................................................... 20!
7! References.......................................................................................................................... 20!
Travel Ordering System
Requirements Specification
Version: 0.1
Project Name <Name>
Project Lead <Name>
Responsible <Name>
Created on <Date>
Last changed
X In process
Submitted
State
Completed
Document File
V-Modell XT Version VMRELEASE 1.3with BISA Extension
illustrative
MetaModelREReferenceModel
Structure Content
Project-specific
Exemplars
instanceofinstanceof
!
PRODUKT.PROJEKTBEZEICHNUNG - PRODUKT.NAME
Zuletzt geändert: 27.10.2010 13:28 3/20
Content
1! Introduction.......................................................................................................................... 6!
1.1! Overview....................................................................................................................... 6!
1.2! Purpose.......................................................................................................................... 6!
1.3! References..................................................................................................................... 7!
1.4! Scope............................................................................................................................. 8!
2! System Vision ...................................................................................................................... 8!
2.1! Summary of Business Specification.............................................................................. 8!
2.2! Scope of Information System under Consideration ...................................................... 8!
2.2.1! System Overview ................................................................................................... 8!
2.2.2! External Systems.................................................................................................. 10!
2.2.3! Use Case Overview.............................................................................................. 10!
2.2.4! Information System Service Overview ................................................................ 10!
3! Information System Requirements..................................................................................... 11!
3.1! Actors .......................................................................................................................... 11!
3.2! Generic Scenarios........................................................................................................ 11!
3.3! Domain-specific Application Capabilities .................................................................. 12!
3.3.1! <<Business Domain>> <Name>.......................................................................... 12!
3.4! Information System Objects........................................................................................ 14!
3.5! System Quality Requirements..................................................................................... 16!
3.6! Architectural Constraints............................................................................................. 16!
3.6.1! Logical Restrictions.............................................................................................. 17!
3.6.2! Technical Restrictions.......................................................................................... 17!
4! Integrational Requirements ................................................................................................ 18!
4.1! Deployment Requirements.......................................................................................... 18!
4.2! Migration Requirements.............................................................................................. 18!
5! Organisational Requirements............................................................................................. 19!
5.1! Project Requirements .................................................................................................. 19!
5.2! Obligations .................................................................................................................. 19!
5.3! Glossary....................................................................................................................... 19!
6! Abbreviations ..................................................................................................................... 20!
7! References.......................................................................................................................... 20!
Travel Ordering System
Requirements Specification
Version: 0.1
Project Name <Name>
Project Lead <Name>
Responsible <Name>
Created on <Date>
Last changed
X In process
Submitted
State
Completed
Document File
V-Modell XT Version VMRELEASE 1.3with BISA Extension
illustrative
MetaModelREReferenceModel
Structure Content
Project-specific
Exemplars
instanceofinstanceof
!
PRODUKT.PROJEKTBEZEICHNUNG - PRODUKT.NAME
Zuletzt geändert: 27.10.2010 13:28 3/20
Content
1! Introduction.......................................................................................................................... 6!
1.1! Overview....................................................................................................................... 6!
1.2! Purpose.......................................................................................................................... 6!
1.3! References..................................................................................................................... 7!
1.4! Scope............................................................................................................................. 8!
2! System Vision ...................................................................................................................... 8!
2.1! Summary of Business Specification.............................................................................. 8!
2.2! Scope of Information System under Consideration ...................................................... 8!
2.2.1! System Overview ................................................................................................... 8!
2.2.2! External Systems.................................................................................................. 10!
2.2.3! Use Case Overview.............................................................................................. 10!
2.2.4! Information System Service Overview ................................................................ 10!
3! Information System Requirements..................................................................................... 11!
3.1! Actors .......................................................................................................................... 11!
3.2! Generic Scenarios........................................................................................................ 11!
3.3! Domain-specific Application Capabilities .................................................................. 12!
3.3.1! <<Business Domain>> <Name>.......................................................................... 12!
3.4! Information System Objects........................................................................................ 14!
3.5! System Quality Requirements..................................................................................... 16!
3.6! Architectural Constraints............................................................................................. 16!
3.6.1! Logical Restrictions.............................................................................................. 17!
3.6.2! Technical Restrictions.......................................................................................... 17!
4! Integrational Requirements ................................................................................................ 18!
4.1! Deployment Requirements.......................................................................................... 18!
4.2! Migration Requirements.............................................................................................. 18!
5! Organisational Requirements............................................................................................. 19!
5.1! Project Requirements .................................................................................................. 19!
5.2! Obligations .................................................................................................................. 19!
5.3! Glossary....................................................................................................................... 19!
6! Abbreviations ..................................................................................................................... 20!
7! References.......................................................................................................................... 20!
Travel Ordering System
Requirements Specification
Version: 0.1
Project Name <Name>
Project Lead <Name>
Responsible <Name>
Created on <Date>
Last changed
X In process
Submitted
State
Completed
Document File
V-Modell XT Version VMRELEASE 1.3with BISA Extension
illustrative
Organisational Level
Process Integration
Project Level
Static Tailoring
Dynamic Tailoring
...
Project Scope
defined
System Specification
accepted
...
Business Analyst
...
Requirements
Engineer
Context
Specification
Requirements
Specification
System
Specification

14. Quality models and dealing with NFRs
• Usage of Quality models in RE
• Exemplary quality models
• Dealing with NFRs in AMDiRE
Dr. Birgit Penzenstadler 14
K Rayker, stock.xchng

15. Excursion: Quality models
Quality models
• Determine which quality aspects and concepts exist and how these are
related
• Support the structured elicita=on and modeling of quality requirements
à E.g. via a taxonomy of quality a7ributes
Examples:
• Classifica=on acc. to Boehm (1978)
• IEEE 29148 Sobware Requirements
Documenta=on Standard (2011)
• ISO/IEC 9126 (1993, revised 2001)
• TUM S&SE Quality Model
Challenges:
– Many different quality aspects and
rela=ons between them
– Systema=cs in their applica=on in RE
(as well as applica=on in assessment)
15
[ISO Std.] (Excerpt)

16. Example: Taxonomy of quality
a7ributes (QuaMoCo)
Ins-tut für Informa-k
Chair IV: So8ware & Systems Engineering 16

17. Assessment of quality models
• Exemplary quality models determine the taxonomy of
quality criteria and a7ributes
Cri=cal aspects
• The models oben stay on the abstract level of „-ili=es“
à No statement about measurability and assessment of
quality criteria
• No direct applicability to RE
– No clues for explicit deduc=on of required system
characteris=cs
Example: The demand for mul=-language documenta=on supports maintainability, but also usability.
– Other non-func=onal aspects are oben not covered
17
But: quality models are the basis for the classifica=on of NFRs.

18. NFR Taxonomy acc. to
Axel van Lamsweerde
18

19. ATM Examples for NFR Taxonomy by
Lamsweerde
19
- Security of
personalized data
- Availability of the
AMT
- ...
The security concept
has to adhere to
IEC 61508 Standard.
(à adequate
architecture)
The ATM uses data
encryp=on.
The full func=onality
shall be available by
May 2014.

20. ISO 25010
• Internal quality: mainly visible in the code, e.g.
reusability of the code within maintainability
• External quality: mainly characteris=cs that
can be seen from outside the system but not
necessarily by the user
• Quality in use: perceived by the user of the
system during run=me
Dr. Birgit Penzenstadler 20

21. ISO 25010

22. ISO 25010

23. Summary
23
What are quality models?
à Conceptual models for the descrip=on of quality.
Usage of quality models in RE
§ Defini=on and assessment of sobware quality – beginning in RE
§ Quality assurance, for example of artefacts in RE
§ Also: Classifica=on of requirements according to characteris=cs
Examples
§ QuaMoCo
§ Lamsweerde taxonomy
§ ISO 25010