The role of human sciences in ergonomics is to provide designers and engineers with information about human characteristics which they can use to ensure that their products 'fit' their users. However, unlike in 'pure' engineering, where one can specify a hole size for a standard screw, the 'one size fits all' approach does not always work for the many and varied members of the human species. Information is required on the variability within the human population

To take a simple example, imagine you are a manufacturer of beds. You know how to make a bed just the right size for you. But you are only 5'2" tall, and you know for a fact that there are taller people out there — your business partner, for example, is a towering 5'9" — and you don't want to exclude them from your potential market, nor to insist on a customer-deterring Procrustean2 solution. So how do you satisfy everyone? Well, you might find out that the tallest person in the UK is Mr Hussain Bissad at 7'7". The only problem is that beds that length wouldn't fit in many bedrooms. So where to draw the line? Well, as it happens, ergonomists have, over many years, compiled collections of anthropometric data— various dimensions measured from a sample of the population. Statistical methods allow us to use these to infer the proportion of the population that would fit a bed of given size. One might decide, therefore, to build a bed long enough for 98% of the population and supply the other 2% as special orders3.

The general aim of ergonomics is to accommodate as much of the population as is appropriate. It is almost never useful to design for an 'average' person, because this person will, by definition, be taller than 50% of the population, but shorter than the other 50%: designing for the average may exclude half of all people. Collections of anthropometric data — which give measurements for all sorts of human parts such as stature, arm length, head circumference, armpit height, hip-knee length etc — typically state means, standard deviations and values for the 5th to 95th percentiles4. It is quite common to design for the 5th percentile to 95th percentile range — although it should be noted that this excludes 10% of the population. Great care must be taken in deciding how big a proportion of the population to accommodate. For example, one might reasonably suppose that pretty much any woman travelling on public transport late at night should be able to reach an emergency alarm button, so this must be brought into 100% reach.

It is not only in physical dimensions that products have to fit humans. For us to use objects effectively, safely and with pleasure, various human characteristics have to be taken into account, such as:

• The size of text required for readability at a particular distance.
• The number of aircraft an Air Traffic Controller can handle without becoming overloaded.
• The number of choices someone can remember from a spoken list.
• The amount of weight that can be carried safely and comfortably.
• The temperature and humidity that a manual worker can endure without becoming heat stressed.
• The complexity of a computer interface that will still let people find the information they need.
• The forces that will stop children being able to undo the tops of medicine bottles.
• The appropriate names to use for menu items so that they are familiar and meaningful to users.

An important role for ergonomists is to identify what characteristics need to be considered and to provide designers with the appropriate human data.