Designing for the 50th percentile is not always appropriate because the “average person” does not represent the full range of users. A fixed median measurement can exclude people whose body dimensions, physical abilities, or needs fall above or below it.
Why the 50th Percentile Can Be Misleading
A percentile indicates the percentage of a population whose measurement is equal to or below a stated value. A person at the 50th percentile for height is therefore taller than, or equal in height to, approximately 50% of the measured population.
However, anthropometric characteristics vary independently. Someone at the 50th percentile for height may be at the 70th percentile for shoulder breadth and the 30th percentile for arm reach. No single person is “average” in every body dimension.
The common misconception is that designing for the median accommodates most users. In reality, one fixed 50th-percentile dimension may be unsuitable for users on both sides of the median.
Selecting Appropriate Anthropometric Data
Under inclusive design, percentile choice should follow the product’s function, with adjustability used where possible.
| Design situation | Appropriate approach | Example |
|---|---|---|
| Reach control | Design for lower percentile | Place controls within reach of smaller users |
| Clearance or access | Design for higher percentile | Provide space for larger users |
| One product for varied users | Use an adjustable range | Chair spanning approximately the 5th to 95th percentiles |
| Varied abilities | Conduct inclusive user testing | Test grips with users who have limited dexterity |
Designers must also match anthropometric data to the target population’s age, sex, geographic origin, and abilities. Data should be combined with user observation, modelling, prototyping, and testing because measurements alone do not reveal comfort, behaviour, or accessibility.