To compare design choices fairly, name your criteria, score every option against each one on the same scale, weight the criteria by importance, and state the choice with the weights shown.
This lesson belongs to SPM subjects and learning resources. It builds on telling an assertion from supporting evidence, because every score should have a reason.
How do I set up the comparison?
Follow four steps.
- List two to four options.
- Choose three or four criteria that matter for this decision.
- Score each option from 1 (weakest) to 3 (strongest) for every criterion. State that 3 always means best.
- Decide weights, then multiply each score by its weight and add.
Worked example: a shopping bag for a school fair
An original case. The school fair committee compares three bags: a cotton tote, a recycled-plastic bag and a paper bag. Criteria are durability, cost and end of life (what happens when it is thrown away or recycled). Scores, with 3 best:
| Option | Durability | Cost | End of life |
|---|---|---|---|
| Cotton tote | 3 | 1 | 3 |
| Recycled-plastic bag | 3 | 3 | 1 |
| Paper bag | 1 | 2 | 3 |
Weighting one: the committee cares most about price. Weights: cost 3, durability 2, end of life 1.
Cotton tote: 3×2 + 1×3 + 3×1 = 6 + 3 + 3 = 12. Recycled plastic: 6 + 9 + 1 = 16. Paper: 1×2 + 2×3 + 3×1 = 2 + 6 + 3 = 11. The recycled-plastic bag wins.
Weighting two: the committee cares most about what happens at the end. Weights: durability 2, cost 1, end of life 3.
Cotton tote: 6 + 1 + 9 = 16. Recycled plastic: 6 + 3 + 3 = 12. Paper: 2 + 2 + 9 = 13. The cotton tote wins.
The scores did not change. Only the weights did, and the recommendation changed with them. That is why weights must be stated.
A sentence you can reuse
“I chose [option] because, with [criterion] weighted most heavily, it scored highest at [total], ahead of [option] at [total]. If [other criterion] mattered most, [other option] would be preferred.”
The mistake to avoid
A common slip is to pick the winner first and then choose criteria that favour it. The table looks fair, but the comparison is not honest.
Fix this by writing the criteria and the direction of the scoring before looking at the options, and by giving a reason for each score.
Check yourself
Compare two school-hall lights. Light P: durability 3, cost 1, energy use 3. Light Q: durability 2, cost 3, energy use 1.
Scores are 3 best. Weights: durability 1, cost 1, energy use 3. Which is preferred, and what would change the answer?
Answer
Light P: 3×1 + 1×1 + 3×3 = 3 + 1 + 9 = 13. Light Q: 2×1 + 3×1 + 1×3 = 2 + 3 + 3 = 8. Light P is preferred.
What would change the answer: if cost weighed 5 and energy use only 1, then P = 3 + 5 + 3 = 11 and Q = 2 + 15 + 1 = 18, so Q would win. The weights carry the decision.
What to study next
Move on to environmental design trade-offs, where no option wins on every criterion. Then practise in the original Sustainability practice set. The structured-answer self-review tool can check that your justification names criteria and weights.
For a teacher to work through new comparisons with you, see online one-to-one Sustainability tuition.