Engineering MakingSeparating Constraints From Criteria in a Design Brief
Atlas stands at a cluttered engineering workbench covered with foam pieces, rulers, cardboard tubes, and sticky notes, sorting red cards labeled MUST into one pile and green cards labeled WANT into another, grinning as he holds both piles up to compare them.
- Explain the difference between a constraint and a criterion in an engineering design brief.
- Identify whether a given design requirement is a constraint or a criterion.
- Compare two design options using ranked criteria while respecting constraints.
- Predict which design option a team should select based on a simple criteria ranking.
Key terms
- Design brief
- The document listing everything a solution must and should do
- Constraint
- A hard pass-or-fail limit a design cannot break
- Criterion
- A ranked goal used to compare designs that pass constraints
- Requirement
- Any stated rule a design must satisfy or pursue
The MUST-Versus-WANT Test
The fastest way to classify any requirement is the acceptance test: ask whether a design could still be accepted if it missed this rule. If missing it disqualifies the design, the requirement is a constraint, a non-negotiable wall. If missing it only makes the design less preferred but still allowable, it is a criterion. This one question resolves almost every confusing case in a design brief.
How Criteria Break Ties
Constraints and criteria do different jobs in sequence. Constraints act first as a filter, removing every design that breaks a hard limit no matter how attractive it is otherwise. Only the surviving designs reach the criteria stage, where you rank goals by importance and choose the option that scores highest on what matters most. Constraints decide who is allowed to compete; criteria decide who wins.
Worked examples
A go-kart brief says: must cost under $40, must seat one rider, and should be as fast as possible. Classify each requirement.
- Test 'costs under $40': a $50 kart is rejected outright, so it is a constraint.
- Test 'seats one rider': a kart that seats nobody fails the brief, so it is a constraint.
- Test 'as fast as possible': a slower kart is still allowable, just less preferred, so it is a criterion.
Answer: Cost-under-$40 and seats-one-rider are constraints; as-fast-as-possible is a criterion.
Activity
Sort each design requirement card into the CONSTRAINT bin or the CRITERIA bin.
Practice
Decide whether 'must be waterproof to 1 meter' is a constraint or a criterion and justify it.
Write one constraint and two ranked criteria for a classroom pencil organizer.
Common mistakes to avoid
- All requirements are basically the sameConstraints eliminate options outright, while criteria only rank the options that already pass every constraint.
- A numeric requirement is always a criterionA number like a maximum cost can be a hard pass-or-fail limit, which makes it a constraint, not a criterion.
Check your understanding
A student's design brief says the model car must travel at least 3 meters and must cost under $5. A second requirement says the car should travel as far beyond 3 meters as possible. Which statement correctly classifies these requirements?
Two shelter designs both pass every constraint. Design A scores higher on 'easiest to build' but lower on 'best wind resistance.' Design B is harder to build but handles wind much better. The team ranked wind resistance as their top criterion. Which design should they choose?
A classmate says: 'Constraints and criteria are basically the same thing — they are both just requirements.' What is wrong with this statement?
Recap
Inside a design brief, constraints are hard limits that disqualify any design that breaks them, while criteria are ranked goals that compare the designs which already pass every constraint, so you filter with constraints then choose with criteria.
Reflect
When have you treated a preference as if it were a hard rule, or the reverse?