measure. For both kinds of measures, a direct involvement of the envisaged users is needed ( Bühler and Schmidt 1993).
Two different concepts of involvement are available: (i) the immediate participation of all users who are or will be influenced in any regard by a new technology+ADs- (ii) the selection of representatives of the prospected target group. Their subjective measures will serve as predictors for the measures of the whole target group. In most cases, the first approach is not feasible due to not precisely defined target groups, size of target groups, restrictions in the resources available for the measures, etc. Unfortunately, also the second approach inherits a variety of risks that may lead to results that do not correctly predict the measures.
Designing for disabled and elderly people, or assessing products developed for these target groups, brings up the problem of recruiting participants for observations and measures in the design or evaluation process. How to design questionnaires for blind users, how to interview people with cognitive impairments or speech disorders, how to motivate elderly persons to try out new technologies, are some of the questions that may arise. As a result, most often economic constraints limit the degree of target group involvement. On the other hand, the question of representativeness of voluntary participants arises. Approaches in the past that involved blind or blindfolded programmers or experiments with students missed the goal and led to wrong conclusions (participants · target group). Testing under simulated disability conditions never substitutes testing with disabled users, because simulation does not represents the rich contexts, experiences and needs of the target group ( Bergman and Johnson 1995). Finally, designing for all means to design for the broadest possible range of users. Still, is it possible to build up an adequate number of user groups and to select appropriate delegates? Creating a number of groups along one dimension of user characteristics, e.g., restrictions in motor abilities, is not a valid grouping for analysing the needs of users regarding other dimensions, e.g., cognitive issues. Unfortunately, only the view of all individual user characteristics at a time builds up a picture of an individual. Obviously, it is difficult to capture the multiple dimensions of individual characteristics. It follows that the participation of user delegates must be carefully designed in order to avoid the reducing of the 'design for all' approach to a 'design for some' approach. For example, a proactive approach, addressing accessibility issues within the design process, is argued to ensure that forthcoming technologies are made accessible to all ( Stephanidis 1995).