While ergonomists can be thought of as the users' advocates in the design process, they will readily admit that they are but second-hand substitutes for the users themselves. In ergonomics, user involvement is regarded as the sine qua non of good design.

An international standard6 has specified the nature of user-centred design. It is aimed at software-based systems, but its principles are equally valid for physical products.

The key is to get future users (or a sample of 'typical' users) involved in a process of 'iterative' design. Iteration — making successive improvements until you get it right — is important because for a new product, users may not know exactly what they want any more than the designers. Take, for example, someone presented with a mobile phone in the early 1980s. How long did it take before people realised they wanted to put them in handbags and that predictive text for a text-messaging system would be useful?

The user-centred design process has the following stages:

• Understand the context of use. Specifying the characteristics of the users and the overall tasks they will carry out. This is where task analysis comes in.
  
• Define the user requirements. Specify what the system or product will do and how it will do it.
  
• Produce user-centred designs. Produce initial designs, bringing in human data where necessary.
  
• Involve users in assessing the designs. Designs must always be evaluated to ensure that they do what they are meant to do in a way that the users are happy with. This need not involve testing a fully-working design. Early evaluations can be made with pen and paper, cardboard models, 'working mock-ups' of web pages, etc.

This cycle should continue until a satisfactory design is reached. Although it might seem like extra work, it can bring savings by allowing design faults to be ironed out at the early stages when they are cheaper and quicker to correct.

The outcome of a user-centred design process will be products which are themselves user-centred. They will be based on evolving user requirements, will match human characteristics and will have been judged satisfactory by users.