1 Requirements: Definition Mark E. Sampson EMIS 8340 Systems Engineering Tool—applying tools to engineering systems.

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Requirements: Definition

Mark E. Sampson

EMIS 8340

Systems Engineering Tool—applying tools to engineering systems

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Requirements Definition

• Once we get all of these requirements captured, now what?

• Analyze• Prioritize• Organize

…the process of getting to the most important requirements (…all requirements are not created equal)

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Requirements Analysis…Requirement Parsers

Automatically looking for requirements…•Structure parsers—most conservative approach•Key-Word parsers—assumes customer wrote the requirements for you•Boolean parsers—is req if shall | will AND be !-maybe•Context parsers—automatic syntactic and semantic analysis

…lots of ongoing researchin this area…

•Automatic news feeds•Indexers•Pattern-matching •Semantic Parsers

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Requirements Analysis…Rationale Capture

• Why? The rationale behind the requirement…

• Why record rationale?• reduces requirement count…you have to justify them• exposes bad assumptions• exposes implementation/pre-selection error• improves communication among stakeholders• shortens/eliminates reviews• captures/maintains corporate/tribal knowledge

[Hooks 2001]

…liver & onions

…an all-metal, trimotor monoplane of maximum Gross of 14,200 pounds, fuel capacity for cruising 1000 miles at 150 mph with at least 12 passengers and crew of two…

TransContinental Air Requirement 1932

Implementation

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Requirements Analysis…Rationale continued…

• What should rationale include• Reasoning• Assumptions• Design choices

•When? When you make decisions, not later. Later never comes.•Where? Associated directly with requirements…so its right with the requirement

Example:Requirement: Height shall not exceed 14 ft. Reason: 99% of interstate highway bridges are greater than 14 ft. Design Choices: Standard steel sheeting materials is 12 ft. Cost of custom material size is prohibitive to go wider. Aluminum sheeting is available in 14 ft sheet from Alcoa.

[Hooks 2001]

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Requirements Definition…Requirement Prioritization

• Some requirements are more important than others.

• Helps you understand your options as things change• Enables you to deliver a useful product as things fall

overboard• Foundation for architectural trades (what can be

postponed…)

...how do you identify which are the most important requirements?

1. Define a scale—1 (critical), 2 (general), 3 (nice to have)…2. Rate/Classify the requirements with stakeholders3. Reconcile differences

[Hooks 2001]

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Requirements Definition…Prioritization continued…

• Kano Analysis……allows you to prioritize customer requirements based on

their impact on customer satisfaction

4-types of customer needs: 1. Surprise/delight factors—

make the product stand out; i.e. active noise suppression

2. More is better—i.e. better fueleconomy

3. Must be—without these you can’tsell the product; i.e. a car thatcan’t meet emissions standards

4. Dissatisfiers—cause the customernot to like the product; i.e. uncomfortable to sit in

[www.isixsigma.com]

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Requirements Definition…Prioritization continued…

• Kano Analysis…continuedThe process:

1. Organize requirements into survey form for stakeholders to find out what kind of requirements they are…

2. Focus on moving the product to the upper-right quadrant

Product should:1. Meet as many of the “must be” features as possible2. Add attractive criteria to put you

above the competition

[www.isixsigma.com]

Satisfiers —requirements which create satisfaction when presentbut experiences no dissatisfaction when it is not present

One-Dimensional —requirements which result in rising satisfaction the more they are fulfilled, but lead to increasing dissatisfaction when less fulfilled.

Must be’s —requirements which do not lead to satisfactionwhen fulfilled, but cause dissatisfaction when not fulfilled

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QFD—House of Quality

What is QFD—method for linking voice of customer to approach to implementation by…• Capturing the voice of the customer (VOC)• Inter-relating what needs to be done with how to do it. • Drives out key requirements for addressing those needs

Statistical Design Institute… Statistical Design Toolkit for Design for Six Sigma, includes a QFD tool.

Introduction/Demonstration by Statistical-Design Institute

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Requirements Definition…Prioritization continued…

• Decision Trees…a way of organizing all items relevent to a decision

• A number of approaches…

A probabilistic approach… 1. Start with decision2. Add all possible solutions3. Add options for each solution4. Add outcome probabilities &

calculate

Tools…Excel (Precision Tree,CrystalBall, Treeage, Visio,…)

[Winston & Albright 2000]

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Requirements Definition…Prioritization continued…

• Decision Trees…continued…

A pair-wise approach for prioritizing

requirements… 1. Start with requirements2. Organized appropriately 3. Add solutions as leafs 4. Pair-wise compare each option5. Rollup

Results: 75% of requirements best met by PC25% of requirements best met by Unix

[Sampson 1996]

SE Workstation

User Issues

Hardware Issues

Software Issues

Network Issues

Integration Issues

Reuse/Communication

Training Issues

Support Issues

Management Issues

Dev. New Legacy

Backups

Emerging Std Compliant

Performance/Response

OS Bias

What SE's have

Limit HW Std's

HW match SW

Proven Workstation

Group Resources

Platform Bias

Big Disk/Storage

Time on StationErgonomics

Pipeline Std.

SW Upgradability

SE SW on UNIX

Existing SW

Terminal Emulation

Mulituser SW ConflictsCustomer Req using...

Mission Critical SWEasy Page Viewing

Network BandwidthLANman BiasSingle Point of Failure

Access to IMS, etc.EE/ME...compatibilityCustomer compatibility

Online Comm.Online InfoTeam Approaches

Learning CurveOnline Doc.Minimum # of Tools

HW/OS SupportApplication Support

AffordableLogistics

Justifiable

UNIX

PCFreedom

UNIX PC

UNIX PC

UNIX

UNIX

PC

PC

UNIX

PC

UNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PC

UNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PC

UNIX

PCUNIX

PCUNIX

PC

UNIX

PC

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Requirements Definition…Prioritization continued…

• Decision Trees…• Sensitivity Analysis…

Results: 75% of requirements best met by PC25% of requirements best met by Unix

How sensitive is the result?• Removing user issues (Freedom) resulted in

70% PC, 30% Unix Workstation tilt• Removing management concerns (ROI, using

existing assets, etc.) resulted in a 60% PC, 40% Unix tilt• With both management & user issues removed

from the decision tree, the results are 50% PC, 50% Unix—i.e. either PC or Unix based solution would meetCustomer requirements equally

[Sampson 1996]

SE Workstation

User Issues

Hardware Issues

Software Issues

Network Issues

Integration Issues

Reuse/Communication

Training Issues

Support Issues

Management Issues

Dev. New Legacy

Backups

Emerging Std Compliant

Performance/Response

OS Bias

What SE's have

Limit HW Std's

HW match SW

Proven Workstation

Group Resources

Platform Bias

Big Disk/Storage

Time on StationErgonomics

Pipeline Std.

SW Upgradability

SE SW on UNIX

Existing SW

Terminal Emulation

Mulituser SW ConflictsCustomer Req using...

Mission Critical SWEasy Page Viewing

Network BandwidthLANman BiasSingle Point of Failure

Access to IMS, etc.EE/ME...compatibilityCustomer compatibility

Online Comm.Online InfoTeam Approaches

Learning CurveOnline Doc.Minimum # of Tools

HW/OS SupportApplication Support

AffordableLogistics

Justifiable

UNIX

PCFreedom

UNIX PC

UNIX PC

UNIX

UNIX

PC

PC

UNIX

PC

UNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PC

UNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PCUNIX

PC

UNIX

PCUNIX

PCUNIX

PC

UNIX

PC