Ergonomics and the relationship between people and products, anthropometric data and percentiles, the use of percentile ranges to size products, and how ergonomic and anthropometric data are gathered and applied in design.

What this dot point is asking

AQA wants you to define ergonomics and anthropometrics, explain anthropometric data and percentiles, justify the use of the 5th to 95th percentile range, and describe how human data is gathered and applied so a product fits, suits and is comfortable for its users.

Ergonomics

Ergonomics is wider than just size. AQA expects you to treat it as four overlapping concerns. Physical ergonomics covers fit, posture, force and repetitive strain, the dimension most often examined. Sensory ergonomics covers how clearly a product communicates through sight, sound and touch, for instance the readability of a display or the audible click of a switch. Cognitive ergonomics covers how easy the product is to understand and operate without error, which links to user-centred design. Environmental ergonomics covers temperature, lighting and noise around the product. Good ergonomics reduces fatigue, error and injury, for example through a well-shaped handle that spreads grip force across the palm, a control panel laid out so the most-used controls fall within easy reach, or a warning tone loud enough to be heard over background noise.

Anthropometrics and percentiles

The single most important skill in this topic is choosing the right percentile for the right kind of dimension, because the logic flips depending on what you are designing.

• Reach dimensions (a shelf height, a control a seated user must touch) are set for the 5th percentile. If the smallest user can reach it, everyone larger can too.
• Clearance dimensions (a doorway, the legroom under a desk, an escape hatch) are set for the 95th percentile. If the largest user fits through, everyone smaller does too.
• Adjustable dimensions (a car seat, an office chair, a monitor stand) span the 5th to 95th range so one product flexes to fit nearly everyone.

A common dimension examined is popliteal height, the vertical distance from the floor to the underside of the knee when seated, which sets seat height. Setting seat height to the 5th percentile popliteal height lets even short users place their feet flat on the floor, preventing the under-thigh pressure that a too-high seat causes. Seat depth is set near the 5th percentile buttock to popliteal length so it does not dig into the back of the knee. This is why a fixed seat is often sized for the smaller user, while back support and armrest height are made adjustable.

Where one size cannot fit everyone (such as an office chair), designers add adjustability so the product fits across the range. Adjustment is more expensive than a fixed size because it adds parts (gas struts, sliders, ratchets), so designers reserve it for the dimensions that matter most to comfort and safety.

Gathering and applying data

Anthropometric data is gathered from large population surveys, such as national sizing studies, and published in tables broken down by sex, age and percentile. Because the same person is rarely at the same percentile for every dimension (someone may be 95th percentile for stature but 30th for grip diameter), designers must not assume an "average person" exists. They select the single relevant dimension for each design decision (reach, grip diameter, clearance, eye height), look up the appropriate percentile for that dimension, and then build and test prototypes with real users drawn from across the range to confirm the predicted fit. Static data (measured standing or sitting still) often has to be combined with dynamic allowances for movement, clothing and posture, which is why a designed clearance is usually set a little above the raw 95th percentile figure.