Requirements: Definition

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EMIS 8390
Systems Engineering Toolapplying tools to
engineering systems
Requirements Definition
UPDATED 9/02/04
Mark E. Sampson
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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 AnalysisRequirement parsers
• Automatically looking for requirements
• Structure parsersmost conservative approach
• Key-word parsersassumes customer wrote
  requirements for you
• Boolean parsersis req if shall will AND be
  !-maybe
• Context parsersautomatic syntactic and semantic
  analysis
• lots of ongoing research in this area
• Automatic news feeds
• Indexers
• Pattern-matching

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• Requirements AnalysisRationale Capture
• Why? The rationale behind the requirement
• Why record rationale?
• reduces requirement countyou have to justify
  them
• exposes bad assumptions
• exposes implementation/pre-selection error
• improves communication among stakeholders
• shortens/eliminates reviews
• captures/maintains corporate/tribal knowledge

liver onions
Implementation
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
Hooks 2001
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• Requirements AnalysisRationale continued
• What should rationale include
• Reasoning
• Assumptions
• Design choices
• When? When you make decisions, not later.
  Later never comes.
• Where? Associated directly with
  requirementsso its right with the requirement.
• Example
• Requirement Height shall not exceed 14 ft.
• Reason 99 of interstate highway bridges are
  greater than 14ft.
• Design choices Standard steel sheeting material
  is 12ft. Cost of custom material size is
  prohibitive to go wider. Aluminum sheeting is
  available in 14ft sheet from Alcoa.

Hooks 2001
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• Requirements DefinitionRequirement
  Prioritization
• Some requirements are more important than
  others.
• Helps to 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?
• Define a scale1 (critical), 2 (general), 3
  (nice to have)
• Rate/Classify the requirements with
  stakeholders
• Reconcile differences

Hooks 2001
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• Requirements DefinitionPrioritization continued
• Kano Analysis
• allows you to prioritize customer requirements
  based on their impact on customer satisfaction
• 4 types of customer needs
• Surprise/delight factorsmake the product stand
  out I.e. active noise suppression
• More is betterI.e. better fuel economy
• Must bewithout these you cant sell the
  product I.e. a car that cant meet emissions
  standards
• Dissatisfierscause the customer not to like the
  product I.e. uncomfortable to sit in

www.isixsigma.com
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• Requirements DefinitionPrioritization continued
• Kano Analysiscontinued
• The process
• Organize requirements into survey form for
  stakeholders to find out what kind of
  requirements they are
• Focus on moving the product to the upper-right
  quadrant
• Product should
• Meet as many of the must be features as
  possible
• Add attractive criteria to put you above the
  competition
• A - Customer requirements which create
  satisfaction the customer is greatly satisfied
  when this element is present but experiences no
  dissatisfaction when it is not present.O -
  One-Dimensional Quality Elements Requirements
  which result in rising satisfaction the more they
  are fulfilled, but lead to increasing
  dissatisfaction when less fulfilled.M - Must-Be
  Quality Elements Requirements, which do not
  lead to satisfaction when, fulfilled but cause
  dissatisfaction when not fulfilled.

www.isixsigma.com
9

• QFDHouse of Quality
• What is QFDmethod 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 with Dr. Jesse
  Peplinski

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• Requirements DefinitionPrioritization continued
• Decision Trees a way of organizing all items
  relevant to a decision
• A number of approaches
• A probabilistic approach
• Start with decision
• Add all possible solutions
• Add options for each solution
• Add outcome probabilities calculate
• ToolsExcel (PrecisionTree, CrystalBall,,
  Treeage, Visio,)

Winston Albright 2000
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• Requirements DefinitionPrioritization continued
• Decision Trees continued
• A pair-wise approach for prioritizingrequirements
  ..
• Start with requirements
• Organized appropriately
• Add solutions as leafs
• Pair-wise compare each option
• Rollup
• Results
• 75 of requirements best met by PC
• 25 of requirements best met by Unix

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• Requirements DefinitionPrioritization continued
• Decision Trees
• Sensitivity Analysis
• Results
• 75 of requirements best met by PC
• 25 of requirements best met by Unix
• How sensitive is the result?
• --Removing user issues (Freedom)
  resulted in 70 PC, 30 SUN workstation tilt
• --Removing management concerns (ROI, using
  existing assets, etc.) results in a 60 PC, 40
  Unix split.--With both management and user
  issues removed from the decision tree the
  results are 50 PC, 50 Unix--i.e., either PC or
  Unix based solution would meet customer
  requirements equally.