"Designing for Multiple Anthropometric Dimensions
There are several body measurements that could be relevant for reaching a touch screen, but a practical one would be Forward Grip Reach distance - roughly the distance from the shoulder axis to the palm of the hand. With those two metrics in mind - eye height and forward grip reach - you could picture any user as the function of two perpendicular lines - a vertical line, representing the individual's eye height, and a horizontal line representing arm reach. This is illustrated above for a range of three different users - note that the wheelchair user has a sitting eye height compared with the two standing users.
While it might seem relatively straightforward as to how to situate the kiosk- place the screen at a distance and height that accommodates the greatest range of users - the story gets more complicated, because, well people are complicated. Not just complicated in a psychological sense, but in an anthropometrical sense as well. The factor that adds complexity is the lack of correlation among anthropometric measurements within people. What do I mean by that? Let's take a step back and think in interaction design terms.
In interface design, one is typically working within the constraints of a display. For example, a common resolution for web browsers is 1024 pixels x 768 pixels. Some older displays might be set at 800x600. So while the specific vertical and horizontal dimensions change, the relationship between height and width, or aspect ratio, remains constant at approximate;y 1.3 in both cases. So if you're taking a design originally intended for 1024x768 and then need to scale it down to 800x600, it will need to be reduced proportionally.
Ergonomic design would be much easier if people had consistent "aspect ratios", but our body measurements are not predictably proportional or strongly correlated. Meaning the that all of the the tallest people in one dimension (such as eye height) do not always have the longest measurement for all other dimensions (for example, forward grip reach). An extreme example, swimmer Michael Phelps has a reach that is longer than the majority of people of the same height. What this means is that for practical purposes, each anthropometric variable could be considered independent of others. (Note that the level of correlation among different metrics can vary - for example, different attributes of the hand are closely correlated to each other, but measurements of different limbs are weakly associated.) So when we are setting an eye height that accommodates the lower 5% to upper 95% of that metric, and then a forward grip reachthat accommodates the lower 5% to upper 95% for that particular metric, we are actually talking about two different groups of people. Only a subset of people who fall within the eye height range will also fall within the reach range, albeit a large subset, but below the 90% of the population we are striving to include.
Another way of understanding this is described in the Herman Miller monograph on The Anthropometrics of Fit. The design focus in this case is fitting people to a chair rather than a touch screen kiosk, but the concept is the same. In the illustration above the back row represents all of the people who were the original intended audience for fitting a chair. Each row in front of that shows how a small percentage of people are excluded with each anthropometric variable (seat height, seat depth, etc.). The front row shows the overlap of all four variables such that "almost one-third of our sample [in blue] had at least one dimension out of four that was either smaller that the 5th percentile female or larger than the 95th percentile male." (Continued via DESIGNING *for humans) [Usability Resources]