[This post is part of our series, Engineering Practices in the Classroom, which explores what engineering practices are, why they matter, and what they look like in real classrooms. Today, we’re focusing on the valuable practice of weighing criteria vs constraints when solving a problem.]
Three middle school students are huddled at their table, deep in thought.
“We’re at $7.75,” one student says. “So that’s under $9.00.” They all look at each other.
The medicine cooler they are designing has a maximum budget of $9.00, so there's room if they want to spend a little more to improve the design. After some discussion, they decide to add aluminum foil to the outside of the box for an extra layer of insulation. They do the math again, adjust, and finally get close to the upper limit: “That would bring us to… $8.75. Okay, that works!” She puts her pencil down, satisfied.
Then another student looks back at the design requirements. “But the cold pack is $0.60,” he says.
The girl’s face changes. “OH. I forgot about that.”
Oops. Now they’re over budget. A flurry of ideas:
“We can’t do aluminum foil then.”
“We could make the box smaller.”
“We could lose a layer?”
“Or! We could decrease the potassium.”
If they reduce the potassium, their solution won’t cool the medicine as much. If they reduce the insulative layers, the box won’t stay cool as long. Or they could keep their full set of features, but make the box smaller—which means it holds less medicine.
Clip: Middle school students work through the budget for their proposed design, and consider trade-offs between cost, box sizes, and amounts of different insulating materials
Notice that as they work, no one on the team asks, “What’s the right answer?" Instead of working a formula to find a specific value, their mental energy is focused on weighing priorities and finding the words to defend tradeoffs as they negotiate what to give up.
The problem criteria describe what success looks like, while constraints define any limits they must work within. Balancing criteria and constraints is one of the most valuable practices that students learn as they work through an engineering challenge.
In the Engineering Medicine Coolers unit, students must think carefully
about how the criteria and the constraints affect each other
Like professional engineers, these middle schoolers know that the project depends on
meeting the goals that will define success for their medicine cooler (get cold and stay cold while holding enough medicine), as well as the constraints that limit their options (cost, materials).
The challenge teaches students to make careful decisions about which factors they want to prioritize.
Criteria and constraints in the real world
Real engineering problems are often very complex. The criteria will depend greatly on the context: who is this solution for and why do they need it? Engineers must work with their clients to create a list of specific criteria for success: What must the solution do? What features are required?
The challenge is that you can't optimize everything at once.
Constraints can be even more complicated. In addition to cost and time limits, engineers must consider limitations that are technical, physical, environmental, legal, and social or cultural.
The challenge is that you can’t optimize everything at once. For any given project, an engineer has to consider give and take. For example:
- A stronger design might be heavier and more expensive.
- A lower-cost design might be less durable.
- A design that is accessible for more people might be more complex to build.
- One design might be strong and easy to build, but require a manufacturing process that creates pollution for a nearby town.
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No matter what an engineer is designing, the criteria and constraints are often very complex.
Every manufactured item we own is the result of many similar decisions and compromises that a designer or engineer made based on the full context of the problem. All of this requires flexible thinking, clear communication, and the ability to explain and defend those choices.
These same skills are critical for life in a complex world. For students, solving a simplified version of this kind of problem helps them formalize and sharpen this kind of problem solving.
In the classroom: preparing students to weigh criteria and constraints
Kids already think about trade-offs in their daily lives. If a child's parent is out of town and he needs to get himself to school, he will do mental calculations.
Criteria for success: Arrive on time, well rested, with all books and supplies.
Constraints: Parents say no rides with strangers, and the school doesn’t allow bikes
He considers his options:
- Ride the bus → reliable but very early pickup, less sleep
- Walk → cold weather, must carry heavy backpack or leave some books at home
- Ride with sister → easier but unreliable timing, potential to be late
Now he is ready to make decisions about what to prioritize: sleeping in, staying warm, or arriving on time? And once he chooses an option—say, riding the bus—he can mitigate the risks by going to bed earlier and setting multiple alarms.
Nearly every student could tell you exactly how they prioritize within constraints like this. However, in an academic setting, problems rarely allow for this kind of nuanced decision-making. The criteria for success on most assignments are more linear: answer these questions, show your work, turn it in. Constraints include time limits and legible writing. Simple.
The tension of competing needs can feel impossible or even unfair.
Because of this "in school" mental framework, they may not recognize that they already know how to do kind of problem solving. When you first present students with an opportunity to weigh constraints against the criteria, the tension of competing needs can feel impossible or even unfair.
Talk about trade-offs from the start
To set students up for success, explicitly name what's going on so they expect this tension. Point out that they already know how to make decisions about trade-offs, and make criteria and known constraints visible early on.
Here's what this might look like at different phases of the Engineering Design Process:
- Ask and Imagine. As a class, create a list of known criteria and constraints. Post the list where everyone can see it throughout the activity and refer back to it often. Also let them know that this list can expand as new questions or problems arise later in the process, leading to new criteria and constraints.
- Plan and create. Guide them with questions like, "Which criteria are the hardest to meet? Does your design hit all of the criteria?"
- Test and improve. Ask about compromises they made: “What did you have to give up to make this part work?" or "How did you decide what was most important?"
Finding the balance
Throughout the design process, we encourage students to use each problem’s criteria and constraints as a metric for success. At the same time, we need to remind students that problems like these almost never have perfect solutions.
Engineers do their best to make informed choices as they work within constraints and narrow their ideas into a workable solution that meets all the requirements. When students experience this part of the engineering process, they begin to understand that real problems in the world involve nuance, adjustments, and compromises.
Ready to try it with your students? Here are a few next steps you can take...
- Download the tip sheet. This free resource includes concrete implementation suggestions.
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Visit our Learning Library. Find more information about this practice.
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Follow YES on social media for more ideas and engineering practices. @yesatmos
Up next in the Engineering Practices in the Classroom series: Considering problems in context
