user requirements phase drawn from sommerville and s. lauesen , software requirements, styles...
DESCRIPTION
User Requirements Phase Drawn from Sommerville and S. Lauesen , Software Requirements, Styles and Techniques , Addisson Wesley, 2002. Overview. User requirements capture and analysis is an early phase of every lifecycle model. - PowerPoint PPT PresentationTRANSCRIPT
1
User Requirements Phase
Drawn from Sommerville and S. Lauesen, Software Requirements, Styles and
Techniques, Addisson Wesley, 2002
2
Overview
• User requirements capture and analysis is an early phase of every lifecycle model.
• Capture means finding out what the user wants … using different dialog techniques … and documenting this.
• Analysis means studying the documented requirements for errors and technical consequences
3
Terminology• Product: the system to be delivered• Inner domain: product + surrounding work area, immediate users,
their activities, other systems• Outer domain: customers, “second-level users”, AKA business domain• Product I/O• Domain I/O• Product-level requirements• Domain-level requirements• Actor: human or external system that communicates with the product• Stakeholder: people who ensure the success of the project. (Not the
same as actors, why?)
4
Terminology
Product
Platform
Other systems
Product I/O
Domain I/O
Inner Domain
Outer orBusiness Domain Actors
Stakeholders
5
Scale of Requirements(responsibility)
Precalculations shall be accurate to within 5%
Goal-level requirement
Product shall support cost recording and quotation with experience data
Domain-level requirement
Product shall have recording and retrieval functions for experience data
Product-level requirement
System shall have screen pictures as shown in App. xx
Design-level requirements
Questions: 1: what can we actually take responsibility for?2: what is the right level of requirement?
6
The Typical URD
1. Introduction: including business goals2. Limits of the system: scope and interfaces3. Data requirements: data model + dictionary4. Product functional requirements: function
lists, feature reqs, process descriptions5. Quality requirements: non-functionalDocumentation standards: PSS-05, IEEE 830
7
Types of Requirements
• Functional requirements: describe what the system does, in terms of input data, output date, error messages, etc.
• E.g. a spreadsheet, a database, a word processor, 3D game, etc
8
Types of RequirementsNon-Functional (AKA Quality) Requirements:• “everything else”• The product• The development process• The system environment
• We can place these in a taxonomy (Sommerville) with a checklist function
• See also:– McCall and Matsumoto (1980)– ISO 9126– IEEE 830 (software requirements specifications)
9
Non-functional
Product Organizational External
Usability
Efficiency
Reliability Portability
speed
throughput
memory
delivery
implementation
standards
legislative
interoperability
ethical
privacy/security
safety commercial
10
Requirements Capture
• An iterative dialog between
• End-usersRequirements
Analysts
Using a variety of tools and techniques
11
Why don’t we just ask the Customer?
• Stakeholders may have difficulty expressing their needs, or may ask for a solution that doesn’t meet their needs.
• Stakeholders can have conflicting demands• Users find it difficult to imagine new ways of doing things,
or to imagine the consequences of what they ask for• A system that fulfills the requirements may not fulfill user
expectations• Sometimes there are no users because a product is
completely new• Demands and the environment change over time
12
Capture techniques
A good analyst:1. knows many techniques,2. knows when to use them and when not,3. Combines and modifies techniques according
tospecific needs.
13
Techniques1. Stakeholder analysis (small scale, who, what, why, risks, costs, solutions?)2. (Group) interview (recorded, taped, filmed)3. Observation (see also ethnography / immersive studies)4. Task demo (“here’s how I usually …”)5. Document studies (company info)6. Questionnaires (large scale, capture statistics & opinions, open/closed questions)7. Brainstorm (unstructured – anything goes)8. Focus groups (structured)9. Domain workshops (business process)10. Design workshops (interface ideas)11. Prototyping (product-level reqs., design-level reqs.)12. Pilot experiments (COTS?)13. Similar companies/ Related products14. Ask suppliers (they know their customers)
14
Example: Organizing a Focus Group
1. Invite participants: 6-18 people, all stakeholders represented, max 30% are suppliers
2. Open the meeting: present the topic, let people get to know each other and relax
3. Bad experiences: roundtable discussion of past experiences with similar products or work domains. Record issues on whiteboard. Record ideas on whiteboard. Facilitator makes sure no one dominates. Supplier staff are low key
15
Focus Group (continued)
4. Imagine the Future: Invite ideas, invite speculation. Ask: why/when do you want this? Record ideas.
5. List the issues: edit on the fly, regroup and organise, combine similar. Record issues.
6. Prioritize issues: Each stakeholder group picks top ten – but don’t prioritize within these to avoid conflict.
7. Review the lists: rountable comment, and close the meeting
16
Requirements Analysis and Validation
• “Are we building the right product”?• i.e. will we build the product the customer
truly wants to have? (at least at some point in time!)
• Paradox: only the customer can determine this … but the customer is non-technical!
17
Requirements Analysis
• Analysis involves several types of checks and tests that can be carried out:
• Validity• Consistency• Completeness• Realism• Verifiability
18
Validity• Problem: User may have incorrectly defined a functional
requirements. All requirements must be checked for functional correctness
• Methods: – Rapid prototype– Paper model– Animation/simulation– Check existing/historic data– Test case generation– URD reviews– System User Manual
19
Consistency
• Problem: User may state requirements that contradict each other (Common with many end-users!)
• E.g. year + 1 > yearyear is a 2-digit number99 + 1 = 00 > 99
contradiction!
Simplified model of the Year 2000 Problem
20
Consistency
• Methods: • If requirements are formal use constraint
solvers and/or CASE tools for automatic check• Manual check, unclear, error prone,
combinatorial explosion!• Note: problem may not be solved by
prototyping
21
Completeness
• Problem: user may have forgotten some requirements, leaving holes in the requirements document. These may possibly be solved arbitrarily … but possibly not!
• Methods: – Rapid prototyping– URD reviews– Test case generation– Use cases analysis– Tables– Fault/ decision trees
22
Realism (Feasibility)
• Problem: User may express requirements that or not technically feasible (e.g. performance) or violate some non-functional requirement (e.g. legislative)
• Methods: – Prototyping– Mathematical model/simulation (e.g queueing theory)– URD reviews– External advice (e.g. lawyers)
23
Verifiability
• Problem: Users may state requirements which can never be checked/verified,
• E.g. “user interface must be user friendly and easy to use”
• Contractual disputes may emerge• Methods:– Test case generation esp. acceptance tests– Usability metrics
24
Numerical Quality RequirementsRequirement Comment
1. Product shall detect license plate and take photo in 0.5 seconds
Physical limits AKA hard-real time req.
2. Product shall compute a room occupation forecast within 2 minutes
Too fast? AKA soft-real time req.
3. Product shall compute a room occupation forecast within 4 minutes
Too slow, supplier makes no effort
4. Product shall compute a room occupation forecast within X minutes.
Open target, but how important?
5. Product shall compute a room occupation forecast within X minutes (customer expects 1 minute)
Open target + expectations
6. Forecast shall be measured 10 times by stopwatch during busy period
Customer might user other approach?
7. Forecast shall be measured by some means specified by customer
Open metric
25
Usability = fit for use + ease of use
The five (ease of) usability factors (Schneiderman 1998):1. Ease of learning2. Task efficiency3. Ease of remembering4. Subjective satisfaction5. Understandability
Some developers claim we cannot optimize all 5, if so … which to prioritize?
26
Usability MetricsMeasure Customer Risk Supplier Risk
Problem Count: At most 1 of 5 novices shall encounter problems during tasks Q and R
low high
Task Time: Novice shall perform tasks Q and R in 15 minutes, experienced user shall complete Q,R,S in 2.
high
Keystroke counts: Recording breakfast shall be possible with 5 keystrokes per guest, no mouse.
medium
Opinion Poll: 80% of users shall find system easy to learn. 60% shall recommend system to other users
medium high
27
Score for understanding: Show 5 users 10 common error messages. Ask for explanation of cause. 80% of the answers shall be correct
medium low
Design-Level Requirements: System shall use screen pictures in app. Xx, buttons work as in app. Yy
high
Product level requirements: For all code fields, user shall be able to select value from drop-down list
medium
Guideline adherence: System shall follow style guide Zz. Menus shall have at most 3 levels.
high low
Development process requirements: Three prototype versions shall be made and usability tested during project.
medium
28
Security Requirements
While other requirements support use-cases ...safety requirements prevent abuse-cases.
Customer has certain assets to be protected against threats.
We will examine security under risk management later …
29
Requirements Capture Languages
• Requirements need to be recorded as precisely as possible,
• Therefore technical requirements languages are useful
• Large variety of these in many styles• We first consider styles: merits and demerits
30
Style: Natural Language
+ easily understood (esp. by end-user)+ no technical training needed+ very high-level/compact requirements- unclear/ ambiguous- debugging is difficult- no inherent structure- no tool support for validation (spell checker?)
31
Style: Structured Natural Language
E.g. tables, decision trees, fault trees, data dictionaries
+ understood by end-user (sometimes)+ small technical training+ some structure (e.g. nouns, verbs, relations etc)+ improve completeness issues- unclear/ ambiguous- lack of standards- little tool support
32
Style: Graphical Requirements Language
e.g. UML, SDL, Petri Nets, etc+ high-level/compact+ quite or very precise+ increasing tool support+ often standardized / multiple vendors, courses,
books, consultants- needs technical training- rarely understood by non-IT people and end-users
33
Style: Formal Specification
Logic: e.g. OCL, JML, VDM, Z, B, temporal logic Ad-hoc: e.g. queuing theory, Markov chain+ good tool support for validation problems+ can be used to generate test cases, prove code
correctness+ extremely precise and accurate- Needs technical training- Poorly understood by end-users- Notation hard to read, overly detailed or low level
34
Data Modeling
• Data models describe data inside and outside the product
• Good for experts, maybe difficult for end-users• Early models can survive all the way to coding• Good for completeness/consistency checking
Options1. Class Diagram (OO analysis) 2. Entity-Relationship diagram (Database theory)3. Data dictionary: terms and meanings4. Data expression: format and legal values. Use regular expressions or DTDs.
35
Tables: a structured style
• Advocated by David Parnas• Informal but structured style• Easily understood by end-users• Many formats, e.g. nested tables• Good for completeness and consistency check• Good for business rules
36
1. Double room used as single 25%
2. Family with more than one room, discount for additional rooms
10%
3. Discount at immediate checkin
a. Before 6pm and hotel less than 50% occupied, fair weatherb. Before 6pm and hotel less than 50% occupied, bad weather
a. 25%b. 0%
Requirement x. The product shall suggest the following discount rates if a customer asks for a discount.