The big idea: The 50th percentile adult or child is the middle value of ONE dimension.
Nobody sits at the 50th percentile of height AND reach AND grip strength AND hearing at once — so designing for the average person designs for somebody who does not exist.
A percentile is a position in a population on one measurement. The 50th is the middle of that one dimension and nothing more.
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Three reasons the 50th percentile misleads
It is one dimension at a time
A person at the 50th percentile for height is very unlikely to be there for arm length, and less likely still for both plus grip strength.
Requirements stack
Three requirements each covering the middle half of users overlap far less than people expect, so a product aimed at everybody fits a small minority.
It hides who is excluded
An average tells you nothing about the tails, and the tails are where products fail people — the smallest reach, the weakest grip, the worst eyesight.
Sometimes it is still right
Where comfort rather than access is at stake and adjustment is impossible — a fixed bus-seat pitch, say — the middle is a defensible compromise. Say why.
What to use instead: Reach, force and readability come from the least capable user you will serve. Clearance, headroom and capacity come from the largest.
Where neither is possible, make it adjustable, or offer sizes — which is what a chair, a bicycle and a car seat all do.
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The statement is explicit that inclusive design is not always possible. A strong answer says WHY in a specific case rather than treating it as a general excuse.
| Where it breaks down | Why | What a designer does instead |
|---|---|---|
| Direct physical conflict | A grip thin enough for a small hand can be too thin for a large one to hold comfortably | Adjustment, or sizes — and say which user the single size favours |
| Safety requires exclusion | A powered saw must NOT be operable by a small child | Deliberate exclusion by force, reach or a lock — stated as a design decision |
| Cost or space is fixed | An aircraft seat cannot be built for the 99th percentile without removing rows | Choose the compromise openly, and provide alternatives where possible |
| The task itself demands capability | Driving requires a level of vision that cannot be designed around | Design the rest of the system — signage, transport, alternatives |
State the exclusion rather than hiding it: If a product cannot serve everybody, the responsible position is to say who it does not serve and why, and to make an alternative available.
An answer that admits an exclusion and defends it is stronger than one that claims a product suits everybody.
How this is tested — how the average correlates to the 50th percentile, and when designing for the average is inappropriate. It comes up two ways:
Paper 1 — multiple choice
- Identify what the 50th percentile actually describes.
- Pick the case where designing for the average is defensible.
Paper 2 — analysing a product
- Explain why designing for the average person fails a named product.
- Evaluate a case where inclusive design is not fully possible.
The trap: Saying the average is "not accurate". It is perfectly accurate — it is a real middle value of one dimension. The problem is that nobody is average in several dimensions at once.
A manufacturer designs a hand tool for the 50th-percentile adult hand. Explain why this is likely to fail, and what should be done instead.
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