The big idea: Design for extremes means setting each requirement from the user at the limit rather than from the average.
Reach, force and readability come from the least capable user you will serve. Clearance, headroom and capacity come from the largest. Everybody between is covered automatically.
The distribution, the collapse of the average band, and the cases where designing for an extreme already shipped.
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Take from the SMALLEST or WEAKEST
- Reach — the shelf, the control, the emergency stop
- Force — the door, the catch, the lid, the trigger
- Readability — text size and contrast
- Comprehension — wording, number of steps
Take from the LARGEST or STRONGEST
- Clearance — doorways, legroom, gaps to pass through
- Headroom — anything overhead
- Capacity — seat width, load rating, handle strength
- Durability — the force a strong user will actually apply
Getting them the wrong way round: Take clearance from the smallest user and large users cannot pass. Take reach from the largest and short users cannot reach.
The test is simple: does the requirement let somebody IN, or does it let something THROUGH? Access requirements come from the least capable; space requirements from the largest.
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| Designed for | Who it turned out to serve |
|---|---|
| Dropped kerbs, for wheelchair users | Every pram, suitcase, delivery trolley, cyclist and person with a bad knee |
| Lever door handles, for arthritic hands | Anybody carrying something, wearing gloves, or opening a door with an elbow |
| Subtitles, for deaf viewers | Roughly a third of all viewers — on commutes, in noisy rooms, in a second language |
| Large high-contrast controls, for low vision | Everybody: faster, fewer errors, in any lighting |
The advantages to state in an answer
Nobody is designed out
Setting the limit from the extreme means the requirement is met for the whole range rather than for a middle band.
One product, not two
No special version to design, tool, stock and support — and no user marked out as needing something different.
The general population benefits
Lower force, clearer contrast and simpler steps are better for everybody, especially when they are tired, rushed or carrying something.
It is usually cheap at design stage
A lever instead of a knob, 3 mm more handle, one instruction per screen. These cost almost nothing to decide and a great deal to retrofit.
How this is tested — the advantages of designing for extremes, and identifying where it has been used. It comes up two ways:
Paper 1 — multiple choice
- Identify which extreme a stated requirement should come from.
- Recognise a product designed using this strategy.
Paper 2 — analysing a product
- Explain the advantages of designing for extremes for a named product.
- Apply the strategy to the requirements of a given product.
The trap: Taking every requirement from the same extreme. Access requirements come from the least capable user; space and capacity requirements come from the largest.
Apply a design-for-extremes strategy to a self-service supermarket checkout, and explain the advantages for the general population.
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