Design Process-4 (concept selection)
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Transcript Design Process-4 (concept selection)
EML4550 - Engineering Design Methods
Concept Selection
Settling on one or more promising ideas to pursue to final
design
Hyman: Chapter 9, Sec. 9.1 & 9.2
Ulrich and Eppinger: Chapters 5 and 6
Dym and Little: Sections 6.1 – 6.3
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1
Concept Development Diagram
Identify
Customer
Needs
Mission
Statement
Establish
Target
Specs
Analyze
Competitive
Products
Generate
Product
Concepts
Select
Product
Concept
Perform
Economic
Analysis
Refine
Specs
Plan
Design/
Development
Project
Action
Plan
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Concept Selection
The ‘Concept Generation’ phase spawned many ideas (good
and bad) and potential solutions to the problem at hand
How do we select from all these competing concepts?
A ‘method’ is needed to systematically weed out poor
concepts and select the best one to proceed with to Final
Design
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Concept Selection Methods
External decision (customers, consultants, etc.)
Product “Champion” (strong personal decision)
Intuition (no rational method)
Pros and Cons (systematic but subjective)
Prototype and test (hardware, expensive and timeconsuming)
Decision matrices (match characteristics vs. pre-specified
and weighted criteria)
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Concept Selection Methods (Cont.)
Although all methods are used in practice, most of the
‘subjective’ methods are very case-specific
Decision matrices represent the most ‘rational’ approach to
concept selection
We will focus this section on the Decision Matrix method
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Concept Selection: Why a Structured Approach?
A customer-focused product (use the needs as guidelines)
A competitive design (do not overlook competing designs)
Better product-process coordination (forces manufacturing issues
into the trade-off)
Reduced time to market (accelerated ‘downselect’)
Effective group decision-making (minimize ‘arbitrary’ decisions and
maximize team exposure)
Documentation of decision process (not lost in someone’s ‘memory’)
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Concept Selection: Why a Structured Approach? (Cont.)
Need to balance:
Desire to have an expedient ‘downselect’
Expediency: proceed to design sooner
Faster time-to market
Less cost of ‘carrying’ many concepts forward
Desire ‘to know more’ before deciding
Risk of making a mistake (pick a loser)
Risk of avoiding a concept because it is new (potential big winner)
Engineers tend to be conservative
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Decision Matrix Method
Stage 1: Concept Screening
Apply an initial ‘filter’ to ‘weed out’ bad concepts and determine
likely ‘winners’. Apply some elements of ‘scoring’ for the purposes of
ranking only
Stage 2: Concept Scoring
Apply weighted criteria to the concepts and proceed with a
quantitative ‘scoring’ system to pick a winner (or winners)
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Decision Matrix Method (Cont.)
Each Stage has 6 steps:
Prepare the selection matrix
Rate the concepts
Rank the concepts
Combine and improve the concepts
Select one or more concepts
Reflect on the results of the process
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Concept Screening
Selection Criteria
Criterion 1
Criterion 2
Criterion 3
Sum “+”
Sum “0”
Sum “-“
Net Score
Rank
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A
B
C
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Screening: Step 1 - Prepare Selection Matrix
Develop a set of criteria
Customer needs (condensed into criteria)
Corporate needs (cost, manufacturing, liability, image, etc.)
Give equal weight to all criteria
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Screening: Step 2 - Rate the Concepts
Give +, -, or neutral rating to each concept based on criteria
Use general notions (no need to get ‘specific’)
Use team consensus (or majority vote)
Refine or split criteria if team consensus is hard to reach
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Screening: Step 3 - Rank Concepts
Add scores and build a ranking
Identify a “benchmark” concept (from ranking or from
external products - competition)
Group the concepts into three categories: “possible
winners”, “neutral”, and “losers”
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Screening: Step 4 - Combine and Improve Concepts
Are we throwing away as ‘loser’ a ‘good’ concept because it
has one or two negatives? Can they be neutralized?
Can two concepts be combined to preserve ‘better than’
qualities while neutralizing ‘worse than’ items? Can we
derive a concept that takes the ‘best of both worlds’?
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Screening: Step 5 - Select one or more concepts
If there is a ‘clear’ winner, then select it and proceed with it
MISSION ACCOMPLISHED
However, usually more than one concept will survive the
screening
Number of concepts to carry forward will depend on
resources and time available
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Screening: Step 6 - Reflect on the Process
Did we achieve consensus?
Were all the team members treated equally? No
trampling?
Did we avoid personal agendas? Department politics?
It is very disruptive to team spirit to ‘drop’ a concept
that someone was championing. Grudges linger for
a long time within a team
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Concept Screening
Selection Criteria
Criterion 1
Criterion 2
Criterion 3
Sum “+”
Sum “0”
Sum “-“
Net Score
Rank
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An example
A
+
0
0
1
2
0
1
2
B
0
+
1
1
1
0
3
C
+
+
0
2
1
0
2
1
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From Screening to Scoring (quantitative)
Selection Criteria
A
% R
A
S
B
R
B
S
C
R
C
S
Criterion 1
Criterion 2
Criterion 3
Score
Rank
Proceed?
R = Rank, S = Score
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Scoring: Step 1 - Prepare Selection Matrix
Using the same selection criteria used in screening give
relative weight to each (must add to 100%)
It is possible to slightly modify the criteria in light of the surviving
concepts
Weights to each criterion are given by team consensus or related to
customer needs
An example
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Scoring: Step 2 - Rate the Concepts
As with screening, give a ‘score’ to each concept based on a
‘quantitative’ (yet still subjective) numbering scale as
follows:
Relative Performance
1 - Much worse than reference concept
2 - Worse than reference concept
3 - Same as reference concept
4 - Better than reference concept
5 - Much better than reference concept
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Scoring: Step 3 - Rank the Concepts
Compute score for each concept
n
s j wi rij
i 1
s j score _ of _ concept_ j
n num ber_ of _ criteria
wi weight _ of _ criterion
rij rating _ of _ concept_ j _ on _ criterion _ i
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From Screening to Scoring
Selection Criteria
Criterion 1
Criterion 2
Criterion 3
Score
Rank
Proceed?
R = Rank, S = Score
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A A B B
% R S R S
10 4 .4 2 .2
30 2 .6 1 .3
60 4 2.4 3 1.8
3.4
2.3
1
3
Y
N
C C
R S
5 .5
3 .9
3 1.8
3.2
2
Y
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Scoring: Step 4 - Combine and Improve Concepts
As before, can concepts be combined to arrive at a better
solution?
Looking at the concepts in the new light of ‘scoring’ can encourage
the team to improve on the initial ideas
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Scoring: Step 5 - Select One or More Concepts
The final selection is never easy
Do ‘parametric’ studies by assigning different weight
distributions and see which concepts come on top on each
try
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Scoring: Step 6 - Reflect on the Process
Are we ready to proceed with the ‘winning’ concept?
Did the method facilitate the selection?
Can the method be improved?
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Concept Selection: Caveats!
Decomposition of product quality
Failure to capture relationship among criteria
Subjective criteria
Methodical, but still high content of subjectivity
Where to include cost
Derived from customer needs, but how about ‘manufacturing’
the product, not all parameters known
Selecting elements of complex systems
Can a complex concept be reduced to a set of simpler concepts?
How about interactions between sub-concepts?
Applying concept selection throughout the development
process
The same approach can apply to the selection of concepts
within a design effort when developing sub-systems of a larger
system
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Concept Selection: Implications to Project
If many concepts are considered, perform a screening
Record results of screening (and criteria used)
Decision matrix for the selection and scores for each
concept
Presentation of the ‘winning’ concept
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