Requirements Engineering
Introduction
(L#1-2)
Qaiser S. Durrani
Last updated (01/12/20)
Software Engineering Phases
1. Vision – focus on why
2. Definition – focus on what
3. Development – focus on how
4. Maintenance – focus on change
Vision Definition Development Maintenance
Requirements Engineering
• Requirement?
• Requirements Engineering?
• Importance of RE
• RE lifecycle
• Requirements Elicitation
• Writing Requirements
• Requirements Prioritization
• Requirements Validation
• Prototyping
• Requirements Management
Requirements Importance
• Buying a home?
• Buying a car?
• Buying a shoe?
• Buying a dress?
• Admit students in a program?
• Project partner?
• Life partner?
Why Requirements?
• Why waste time worrying about
requirements?
• Why not save money by eliminating
this unnecessary step?
Because - 1
• The later in the development cycle that
a software error is detected, the more
expensive it will be to repair
• Many errors remain latent and are not
detected until well after the stage at
which they are made
• There are many requirements errors
being made
Because - 2
• Errors made in requirements
specifications are typically incorrect
facts, omissions, inconsistencies,
and ambiguities
• Requirements errors can be
detected
Potential Impact of
Requirements Errors - 1
• The resulting software may not satisfy
user’s real needs
• Multiple interpretations of requirements
may cause disagreements between
customers and developers, wasting time
and money, and perhaps resulting in
lawsuits
Potential Impact of
Requirements Errors - 2
• It may be (surely is) impossible to
thoroughly test that the software
meets its intended requirements
• Both time and money may be
wasted building the wrong system
• Impact on humans
• Studies such as the CHAOS report [Johnson 2000]
indicate that about half of the factors associated with
project or product success are requirements related.
Researchers have reported on studies showing that
project success is directly tied to requirements quality
[Kamata et al. 2007].
• Question is why is it still a relatively neglected topic
in university training?
• It is quite rare, for example, that a new Computer
Science (CS) university graduate might be asked to
participate in the development of a compiler or
operating system, yet nearly every graduate working
in the industry will, sooner or later, be asked to
participate in creating the requirements specifications
for a product or service.
Errors introduced during
Requirements activities accounts
40-50% of all software defects found
in software product (Davis 2005)
Requirements Engineering
Customer Statement:
“I know you think you understand
what I said, but what you don’t
understand is what I said is not
what I mean.”
Software Requirements?
(IEEE)
1 A condition or capability needed by user to
solve a problem or achieve an objective.
2 A condition or capability that must be met
or possessed by a system or system
component to satisfy a contract, standard,
specification, or other formally imposed
document.
3 A documented representation of a
condition or capability as in 1 or 2.
Software Requirements?
The statement of needs by a user that
triggers the development of a program
or system (Jones).
Software Requirement?
Requirements are ... A specification of
what should be implemented. They are
descriptions of how the system should
behave, or of a system property or
attribute. They may be a constraint on
the development process of the system.
(Sommerville 1997)
The Requirements Phase
(traditional)
• Begins
– There is a recognition that a problem
exists and requires a solution
– A new software idea arises
• Ends
– With a complete description of the
external behavior of the software to
be built
Requirements Engineering - 1
• One of the most critical processes of
system development
• Iterative system development - process
repeated until requirements are of
acceptable quality
Requirements Engineering - 2
• Requirements elicitation
– Determining what the customer requires
• Requirements analysis and negotiation
– Understanding the relationships among
various customer requirements and
shaping them
• Requirements specification
– Building a tangible model of requirements
Requirements Engineering - 3
• System modeling
– Building a representation of requirements
that can be assessed for correctness,
completeness, and consistency
• Requirements validation
– Reviewing the model
• Requirements management
– Identify, control and track requirements
and the changes that will be made to them
Requirement Activities
• Elicitation: Learning,
uncovering, extracting and
discovering needs of
customers and potential
stakeholders
• Modeling: Creating &
Analyzing models of
requirements for increasing
understandability and search
for incompleteness,
correctness, inconsistency,
etc.
• Triage: Which subset of
requirements are appropriate
to be addressed in specific
releases.
Requirement Activities …
cont
• Specification:
Documentation of desired
external behavior of system
• Verification: Determining
consistency, completeness
and other defects in a set of
requirements
Requirement activities are not
performed sequentially, but in
parallel.
Specification Levels
• Requirements Definition
– Natural Language Statements
– Services expected from System
– Client Understandable
• Functional Specifications
– Structured document – sets out the system
services in detail
– Contract between system procurer and
developer
Specification Levels
• Software Specifications
– Abstract description of S/W – basis
for design and implementation
– Use formal specification techniques
Software Requirements Specification
(SRS)
• A document containing a complete
description of what the software
system will do without describing how it
will do it.
• May suffer from “what versus how”
dilemma.
Levels of Requirements
• Business Requirements
– Represent high level objectives of the
organization or customer requesting the
system or product
– Captured in a document describing the
project vision and scope.
• User Requirements
– Describes tasks the user must be able to
accomplish
Levels of Requirements
• Functional Requirements
– Define the software functionality the
developers must build into the
product to enable users to accomplish
their tasks - thereby satisfying the
business requirements.
Levels of Requirements
• Non-Functional Requirements
– Include regulations, standards, and
contracts to which the product must
conform:
• Description of external interfaces
• Design and implementation constraints
• Quality and performance attributes.
– Constraints are restrictions that are placed
on the choices available to the developer for
design and construction of the software
product.
Levels of Requirements
• All user requirements must align
with business requirements
• Requirements do not include
design or implementation details.
=> Focus on what to build.
Kinds of requirements
(Example)
• BR – user will be able to correct spelling errors in a document
efficiently
– Spell checker is included as a feature
• UR – finding spelling errors in the document and decide whether
to replace each misspelled word with the one chosen from a list
of suggested words
• FR
– Find and highlight misspelled words
– Display a dialog box with suggested replacements
– Making global replacements
• NFR – It must be integrated into the existing word-processor
which runs on windows platform
– Auto vs. manual correction (user perspective)
Relationships among several types of
requirements information
Potential stakeholders within the project team
Stakeholders participation in Requirements Development
Benefits from a high-quality
requirements process
• Fewer defects in requirements and
in the delivered product
• Reduced development rework
• Faster development and delivery
• Fewer unnecessary and unused
features
• Lower enhancement costs
Benefits from a high-quality
requirements process ..
• Fewer miscommunications
• Reduced scope creep
• Reduced project chaos
• Higher customer and team member
satisfaction
• Products that do what they’re
supposed to do
Requirements Pulse
• The project’s business objectives, vision, and scope
were never clearly defined.
• Customers were too busy to spend time working with
analysts or developers on the requirements.
• Your team could not interact directly with
representative users to understand their needs.
• Customers claimed that all requirements were critical,
so they didn’t prioritize them.
• Developers encountered ambiguities and missing
information when coding, so they had to guess.
• Communications between developers and
stakeholders focused on user interface displays or
features, not on what users needed to accomplish
with the software.
Requirements pulse …
• Your customers never approved the requirements.
• Your customers approved the requirements for a
release or iteration and then changed them continually.
• The project scope increased as requirements changes
were accepted, but the schedule slipped because no
additional resources were provided and no functionality
was removed.
• Requested requirements changes got lost; no one knew
the status of a particular change request.
• Customers requested certain functionality and
developers built it, but no one ever uses it.
• At the end of the project, the specification was satisfied
but the customer or the business objectives were not.
References
• Software Requirements by Karl
Wiegers, 2013
• Requirements Engg Processes and
Techniques by Kotonyo &
Sommerville
