Anthropometry is to guide design sizing to fit people. As design becomes more sophisticated, it has become clear that product quality depends on ease-of-use as well as on traditional properties like durability and aesthetics. The various dimensions that users interact with in a product are an important part of usability, affecting factors like grip, reach, comfort and manipulability.

The ability of the human body to adapt can make even badly mis-sized products seem usable, but user satisfaction is always higher for ergonomically sound products than for others. It is the difference between being easy to use and merely possible to use, and ergonomics is now an essential part of high quality design.

Although it is easy to select sizes that suit the designer, it is much more difficult to choose dimensions that are optimised for the whole range of people who will eventually use the product.

It is important not to design for 'the average', in the belief that this will generally give the best compromise for a fixed-size dimension. In fact in most cases, either big people will have more difficulty than small people, or vice-versa. For example, if users have to reach something, designing to the average reach will exclude half of them! If the users have also to fit inside some other part of the design, designing for the average will exclude the other half, leaving a design which seems suited to 'most average people' but fits nobody at all! We have seen seats which have exactly this problem.

Another frequent mistake is to base sizing on one or two individuals, often people who are influential in the organisation. This creates three sources of error:

1. Individuals vary widely in their proportions, so that a tall man may have, for example, long legs and a short back. If you use such a person to decide a car roof height, you will end up with a roof that is too low.
2. The individuals may not represent the customers at all. It is very unlikely that a few people will contain the largest and smallest customers (who are by definition quite rare), and successful people as group are taller and leaner than the population as a whole, and this tends to bias many designs towards fitting taller people better than they do shorter people.
3. Individual subjective decision-making can result in decision-makers' creating products that don't even fit themselves, by building in a slouch or some other personal quirk.

The answer to these problems is objectivity. One of the reasons that sizing is so often approximated is the sheer time and effort it takes to find data:

You or your tutor may like this free Student anthropometry Practical - The Thinking Drinkers Bar, originally developed for use with PeopleSize as part of the ergonomics course at Loughborough University.

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