Documenting Design Decisions

Lesson

Judges often ask a simple question: why did you build it this way? A team that can answer with evidence shows real engineering. A team that says it just looked good misses the chance to show its thinking. Documenting design decisions means writing down the problem, the options, the comparison and the reason for your choice at the time you make it.

The engineering design process. Each step leaves a record in your log that you can later use in the portfolio.

Start with the problem

What must the mechanism do? For example: pick up a game element from the floor.
Constraints: size, weight, motor or servo count, budget, build time. Check the current manual for limits.
Success measure: how you will know it works, such as picking up the element in under 2 seconds in 9 out of 10 tries.

Record the options you considered

Brainstorm several ideas before building anything. Sketch each one, note where the idea came from, and write one or two lines about its likely strengths and weaknesses. Keep the rejected ideas in your log. They are proof that your final choice was a decision, not an accident.

Build a decision matrix

1
Choose criteria
Pick 4 to 6 things that matter, such as speed, reliability, ease of building and weight.
2
Set weights
Give each criterion a weight, for example 1 to 3, based on how important it is for your strategy.
3
Score each option
As a team, score every option on every criterion, for example 1 (poor) to 5 (excellent).
4
Calculate totals
Multiply each score by its weight and add up the results for each option.
5
Discuss the result
Check whether the winner makes sense. If the team disagrees, revisit the scores or weights and write down why.

Criterion (weight)	Roller intake	Claw gripper	Scoop
Speed (3)	5 x 3 = 15	3 x 3 = 9	4 x 3 = 12
Reliability (3)	4 x 3 = 12	4 x 3 = 12	2 x 3 = 6
Ease of building (2)	3 x 2 = 6	4 x 2 = 8	5 x 2 = 10
Weight (1)	3 x 1 = 3	4 x 1 = 4	4 x 1 = 4
Total	36	33	32

Write the rationale

TEXT

1Decision: Intake type for collecting game elements
2Date / decided by: 2 Oct, build sub-team + captain
3Options considered: roller intake, claw gripper, scoop
4Chosen: roller intake
5Why: highest score on speed and reliability, our top priorities
6Risks: more parts to build; needs a motor
7What would change our mind: prototype fails to grab elements
8  from the side in testing
9Evidence: decision matrix (log p. 14), prototype photos

A simple rationale template you can copy into your log.

Write the rationale on the day you decide, while the reasons are fresh. Include who was involved and what result would make you change your mind. That last line is powerful: it shows judges you are ready to learn from testing instead of defending a choice no matter what.

Key terms

Problem statement: A short description of what a design must do, the constraints it must meet and how success will be measured.
Constraint: A limit the design must respect, such as size, weight, budget, build time or a rule in the manual.
Decision matrix: A table that scores several options against weighted criteria to help a team compare them fairly.
Rationale: A written explanation of why the team chose an option, including evidence and known risks.

Practical exercise

Document one real design decision

1Choose a design question your team faces this week, such as which intake or lift to build.
2Write a problem statement with what it must do, the constraints and a measurable success goal.
3Sketch at least three options and write one strength and one weakness for each.
4Build a weighted decision matrix with 4 to 6 criteria and score each option as a team.
5Write a rationale using the template, including what result would change your mind.

Deliverable: A log page with the problem statement, three sketched options, a completed decision matrix and a written rationale.

Knowledge check

Answer every question, then check your answers.