Reach, clearance, and adjustability matter because products must accommodate the physical dimensions and capabilities of their intended users. Applying these ergonomic principles improves safety, comfort, accessibility, and usability while reducing user fatigue and injury.
The Reasoning
Designers use anthropometric data, which are measurements of human body dimensions, to determine appropriate product sizes. Because users vary in size, designing only for the average person excludes many people.
| Principle | Design approach | Why it matters |
|---|---|---|
| Reach | Use data from smaller users, often near the 5th percentile of the target population | Ensures controls, handles, and objects can be reached without excessive stretching |
| Clearance | Use data from larger users, often near the 95th percentile | Provides enough space for users to enter, fit within, or move around a product safely |
| Adjustability | Provide a range that may accommodate approximately the 5th to 95th percentiles | Allows one product to suit users with different body dimensions, preferences, and physical needs |
For example, a car seat must allow a smaller driver to reach the pedals and steering wheel. At the same time, the vehicle must provide sufficient headroom and legroom for a larger driver. An adjustable seat, steering wheel, and head restraint help accommodate both users.
These principles must be applied to the relevant target population. Anthropometric data vary according to factors such as age, sex, geographical population, and physical ability, so one dataset is not universally appropriate.
A common misconception is that designing for the 50th percentile creates a product suitable for everyone. It only suits users near the median and may provide inadequate reach for smaller users or inadequate clearance for larger users.
Exam Technique
In an IB Design Technology response, identify the relevant principle, explain which percentile data should guide the design, and connect it to a specific feature. For an question, include the consequence for the user, such as improved safety, comfort, or accessibility, rather than merely defining the term.