Lesson 1.4: Choosing Between Designs With a Weighted Decision Matrix
Technical Context
Design arguments inside a team are usually won by whoever is most persistent, most senior, or most willing to keep talking. None of those correlate with being right.
A weighted decision matrix does not remove judgment. It makes the judgment explicit, so the team is arguing about criteria and weights, which is a productive argument, rather than about conclusions, which is not.
How It Works
- List the criteria that actually matter for this decision.
- Weight each criterion from 1 to 5 by how much it matters.
- Score each option from 1 to 5 against each criterion.
- Multiply and sum. The weighted total ranks the options.
The critical rule is ordering: weight the criteria before you score the options. Doing it the other way lets the team quietly tune the weights until the favorite wins, which produces a matrix that documents a bias rather than a decision.
Choosing Criteria That Are Not Redundant
Weak matrices list five criteria that are all the same criterion in different words. "Fast," "quick cycle," and "efficient" will be scored identically and triple-count one factor.
Criteria that tend to be genuinely independent for FTC mechanisms:
| Criterion | What it captures |
|---|---|
| Cycle time | How fast the mechanism completes its job |
| Reliability | How often it works out of many attempts |
| Build difficulty | Hours, tooling, and skill required |
| Weight | Mass added to the robot |
| Serviceability | How fast it can be repaired between matches |
| Packaging | Whether it fits the size envelope and clears other subsystems |
| Cost | Parts you have to buy |
Pick four to six. More than six and the weighted totals converge, because everything ends up mattering a little.
If your strategy is high cycle count, cycle time gets a 5 and weight might get a 2. If your strategy depends on a heavy arm reaching far, weight gets a 5. The weights are where your strategy enters the mechanical decision, and they should differ between teams with different strategies.
Try It
The matrix below is pre-filled with three intake concepts. Change the criteria, the weights, and the scores to match a real decision your team is facing.
Weighted Decision Matrix
Score each option 1 to 5 against criteria your team weighted first.
A clear leader. Record the matrix in the notebook with one sentence per rejected option explaining what lost it.
| Option | Cycle time (x5) | Build difficulty (x3) | Reliability (x5) | Weight (x2) | Score |
|---|---|---|---|---|---|
| 60 (80%) | |||||
| 65 (87%) | |||||
| 39 (52%) |
Passive wedge intake wins by 5 points. Write the matrix into the notebook along with one sentence on why each losing option was set aside. That sentence is what judges ask about.
Weight the criteria before you score the options. Doing it the other way around lets the team quietly tune the weights until the favourite wins, which produces a matrix that documents a bias instead of a decision.
Reading the Result Honestly
A matrix output is not a verdict. Read it in three steps.
If one option wins by a wide margin, that is a real signal. Record it and move on.
If the top two are within a few points, the matrix has told you something useful: these options are close, and the scoring noise is larger than the difference. Prototype both. This is the correct outcome, not a failure of the method.
If the winner surprises everyone, do not immediately adjust the weights. First check whether the surprise comes from a criterion that was weighted higher than the team actually believes. If the weights were honest, the surprise is information.
Scoring "reliability" for a concept nobody has built is a guess wearing a number. That is acceptable for an initial screen, but it is not evidence. After prototyping, re-score the matrix with measured values and see whether the ranking survives. In Module 10 you will collect the data that makes those scores real.
What Goes In the Notebook
Photograph or redraw the completed matrix, and add one sentence per rejected option explaining what specifically lost it. "The claw scored lowest because its 4 second alignment time made the cycle 40 percent slower than the requirement allowed" is the sentence judges ask about.
Fill-in-the-Blank Practice
- Criteria must be weighted
__________the options are scored, so the weights cannot be tuned to favor a preferred outcome. - When the top two options finish within a few points of each other, the correct next step is to
__________both. - Scoring a concept for reliability before anything has been built produces a
__________rather than evidence.
Show answers
- before
- prototype (build rough versions and test)
- guess (an estimate, not measured data)
Exercise
Take the two surviving concepts from the Module 0 exercise and score them properly. Weight the criteria as a full team before anyone sees the scores. Record the result in the notebook with the rejection sentences.
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