Professional Practice Skills

PPS-4: Developing Criteria and Constraints

(Adapted from MPS 23, Don Woods 2003)

 

Pre-class assignment

  1. Read sections  What is It?, New Concepts, Why Do It?, How to Do It, and Learning Objectives
  2. Establish your Baseline on this skill on the Feedback Form.
  3. Be able to define the criteria and constraints when asked in class and describe two ways of developing criteria

 

What is It?

A dictionary definition of Criterion is “a standard on which a judgment or decision may be based”.  To be useful for making judgments or decisions, the criteria must be measurable.  

 

Criteria can be viewed in four categories

 

The criteria should be set before making a decision, or even before developing options for a decision.  This prevents the tendency among humans to make a decision first and rationalize it afterward.

 

New Concepts

Criteria, Constraints, Measurability, Checklists

 

Why Do It?

Measurable Criteria are what turn vague goals into clear objectives.  In PPPS-3, we talked about making goals unambiguous and measurable so that we could determine if those goals were achieved.

 

Decision making is another important area in which measurable are needed.  We will use criteria to help make decisions such as what is the “best” design choice, or which of the ideas from a brainstorming session are the most useful. 

 

Decisions on what movie to see, what car to buy, or who to invite to the wedding can also benefit from clear goals and explicit criteria.  For decisions like this, the measurables will be harder to quantify numerically, but relative comparisons can still be made. 

 

How to Do It

Keep your eyes on the prize

Sometimes the forest of criteria will obscure your view of the goal.  Remember that criteria are the servants of the goal, not the masters.  Now, before we drown in a torrent of metaphors, let’s look at how we can come up with those criteria.

 

Checklists

A good way of developing criteria is to use a checklist to generate the important factors; then turn those factors into criteria by making them measurable.  Many different checklists are available.  Design checklists can be found in your design textbooks, handbooks, and on vendor’s websites.  Ergonomic, environmental, and safety checklists are also available.  You can, of course, generate your own checklist.

 

A first step in selecting a checklist is to decide at what level in the design process you are, and make your list match the level of required detail.  For example, at the beginning of the process, you may want to look at a big picture list like the following:

Later you may be looking at specifics and want a checklist like the one available for plastic selection shown on Page 6.

 

Achieve, Preserve, Avoid

Another way to develop criteria for a goal is to answer the questions:

The answers to those questions are the issues that will become the criteria.

 

For example, suppose you are considering the decision to replace your 10 year-old minivan with a new vehicle.  You may want to Achieve greater reliability, Preserve the carrying capacity for people and stuff, and Avoid obnoxious car salespeople.

 

Make it Measurable

Once you have the factors or issues, you need to turn them into criteria by making them measurable.  For example, suppose we are looking at the decision to replace the minivan.  If reliability is the issue, a measurable criterion could be: “The new vehicle must rate at least a red half dot in the Consumer Reports reliability scale.” 

 

For another example, consider selection of a material for a Hip Implant.  One goal is for the material and bone to have a similar Young’s modulus (elastic modulus).  The criterion may read, “The elastic modulus of the stem of the implant should be within 15% of the modulus for cortical bone.” 

 

 

 

Learning Objectives

You should be able to:

  1. Define and give examples of terms listed in New Concepts
  2. Evaluate criteria to determine if measurable
  3. Generate criteria using checklists or the Achieve, Preserve, Avoid approach.
  4. Given a statement of goals, identify stated criteria & constraints and inferred criteria & constraints

 

 

In-Class

Exercise 1 (5 min.): As part of a small group, rate the following statements on measurability (Scale of 1 to 5 with 1 as unmeasurable to 5 as clearly measurable)

            A good pickup truck should

_         Have good carrying capacity

_         Have four wheel drive

_         Be affordable

_         Have good low end torque

_         Get reasonable mileage

_         Be colored red

_         Have room for 4 people

 

Exercise 2 (8 min.): As part of a small group, rewrite two of the least measurable factors to be measurable criteria.

 

Exercise 3 (5 min): As part of a small group, read the following statements, and write down the explicit and implied criteria.  Identify which are constraints.

 

After Rose, I want to study Biomedical Engineering in a warm place with a good football team.  Of course, they have to pay a reasonable stipend and have a tuition waiver.

Explicit                         Implied                         Constraint

 

 

 

We are getting too many warranty returns of these three hole punches.  Management is ticked about the warranty costs and afraid we’ll lose market share due to unreliability,  Metallurgy tells us the failures are primarily due to fatigue.  Redesign this, would you?

Explicit                         Implied                         Constraint

 


Criteria and Constraints Feedback Form

 

Name _______________________                     

 

1.       At the outset of this unit, place a “B” in each category to indicate your self assessment of your initial, or baseline skill level.

2.       At the end of the unit place an “A” in each category to indicate your self assessment of your skill level after practicing the skill.  Be prepared to provide documentation for your assessment.

 

Novice

(less successful)

Beginner

(shows few expert behaviors)

 

(1-2)

Good Start

(some expert behavior)

(3-4)

Getting There

(many   expert behaviors)

(5-6)

Almost There

(mostly expert behavior)

(7-8)

Expert

(shows all expert behavior)

 

(9-10)

Expert

(more successful)

Make a decision and then find criteria to support the decision

 

 

 

 

 

Establish criteria and constraints before a decision

Pick criteria at random

 

 

 

 

 

Use organized approach to establishing criteria

(standards, checklists, Achieve, Preserve, Avoid)

Criteria sound good but can’t be quantified

 

 

 

 

 

Criteria are clear and measurable

 

 

Reflection

What did I learn from this?

 

 

 

Which of the skills do I do pretty well?  (List Evidence)

 

 

 

Which skills could use some work? (List Evidence)

 

 

 

 


PPS-4  Criteria and Constraints

Assignment 1 - Individual

As an individual, you will develop the important issues for a decision, categorize them, and write them as measurable criteria.

 

Tasks:

  1. In the following table write the most important 7 to 10 issues you would want to Achieve, Preserve and Avoid if you were selecting a new place of residence.

 

Achieve

Preserve

Avoid

 

 

 

 

 

 

 

 

 

  1. Place the Issues listed in Task 1 into the following table

 

Must

Want

Don’t Want

People

 

 

 

 

 

 

 

 

 

 

  1. Rewrite your top four issues as measurable criteria in order of importance

 

 

 

 

 

 

Evaluation:

1. Generated issues

Excellent (10)  - 7-10  good issues listed in Achieve, Preserve, Avoid categories

Mediocre (5)  - < 3-4 good issues,

Weak (0) -  no issues

2. Placed in categories of Must, Want, Don’t Want, People

Excellent (10) – Issues were in appropriate categories

Mediocre (5)  - some Issues were in appropriate categories

Weak (0) – missing or all incorrect categories

3. Criteria were Measurable

Excellent (10) – All four criteria were measurable

Mediocre (5)  - Some criteria were not easily measurable

Weak (0) – Criteria were not measurable

 


PPS-4   Criteria

Assignment 2: Group Task

 

In PPP-1, Assignment 1, your group brainstormed solutions to a problem specific to your design.  For that problem, you will use a checklist to develop criteria for evaluating your brainstormed ideas.

 

Task

  1. Use the checklist below to generate factors for your design
    1. Safety
    2. Cost
    3. Function
    4. Restrictions on size, weight, or volume
    5. Reliability, maintainability, availability, and repairability
    6. Candidate materials and manufacturing processes
    7. Quantity to be produced
    8. Stress or load considerations
  2. Prioritize the top 5 or 6 issues and rewrite them as measurable criteria

 

 

Turn in:

On a plain white or engineering problems paper (neatly handwritten or typed)

  1. List of factors generated from Task 1
  2. Prioritized top 5-6 criteria from Task 2

 

Evaluation:

List of Factors

Excellent (10 pts) –  Significant list of design factors that relate to checklist

Mediocre (5 pts) – half-hearted list of issues that may not relate to checklist

Weak (0 pts) - missing list

 

Criteria

Excellent (10 pts) –  5-6 clearly important measurable criteria

Mediocre (5 pts) – fewer than 5, or not all clearly important, or not all measurable

Weak (0 pts) - fewer than 3, or most not important, or most not measurable

 


DESIGN CHECKLIST for Polymer parts (by Ticona)

These are the questions the product development team needs to answer before the right plastic material can be selected:

 

1.   What is the function of the part?

2.   What is the expected lifetime of the part?

3.   What agency approvals are required? (UL, FDA, USDA, NSF, USP, SAE, MIL spec)

4.   What electrical characteristics are required and at what temperatures?

5.   What temperature will the part see? And, for how long?

6.   What chemicals will the part be exposed to?

7.   Is moisture resistance necessary?

8.   How will the part be assembled? Can parts be combined into one plastic part?

9.   Is the assembly going to be permanent or one time only?

10. Will adhesives be used? Some resins require special adhesives.

11. Will fasteners be used? Will threads be molded in?

12. Does the part have a snap fit? Glass filled materials will require more force to close the snap fit, but will deflect less.

13. Will the part be subjected to impact? If so, radius the corners.

14. Is surface appearance important? If so, beware of weld lines, parting line, ejector location, and gate vestige.

15. What color is required for the part? Is a specific match required or will the part be color coded? Some glass or mineral filled materials do not color as well as unfilled materials.

16. Will the part be painted? Is a primer required? Will the part go through a high temperature paint oven?

17. Is weathering or UV exposure a factor?

18. What are the required tolerances? Can they be relaxed to make molding more economical?

19. What is the expected weight of the part? Will it be too light (or too heavy)?

20. Is wear resistance required?

21. Does the part need to be sterilized? With what methods (chemical, steam, radiation)?

22. Will the part be insert molded or have a metal piece press fit in the plastic part? Both methods result in continuous stress in the part.

23. Is there a living hinge designed in the part? Be careful with living hinges designed for crystalline materials such as acetal.

24. What loading and resulting stress will the part see? And, at what temperature and environment?

25. Will the part be loaded continuously or intermittently? Will permanent deformation or creep be an issue?

26. What deflections are acceptable?

27. Is the part moldable? Are there undercuts? Are there sections that are too thick or thin?

28. Will the part be machined?

29. What is the worst possible situation the part will be in? (For example, the part may be outside for an extended period of time and intermittently put in water, or the part may see a constant high load while submerged in gasoline at 150°F.) Parts should be tested in the worst case environment.