Part 1 of E *IxD set up the conceptual background on why ergonomics is a valuable knowledge area for interaction designers and discussed some of the basics of anthropometrics (designing for fit). We were looking at Eye Height as a critical attribute for positioning the height of a kiosk display, so that a broad range of users could comfortably view the screen. But having the display at an appropriate height for visibility is just addressing one aspect of interaction - the user also needs to control the interface - in this case via a touch screen.
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."
There are some analytical methods for more effectively addressing these issues mathematically, but that's beyond the scope of discussion (for those interested, see Guidelines for Using Anthropometric Data in Product Design) . In practical terms there are three solution approaches: design multiple sizes, adjustability and satisficing.
Multiple sizes, as it implies, creates a range of models, where each is targeted at a specific subset of the user population. The most extreme example of this (aside from bespoke, individualized designs) comes from clothing and footwear, where there are literally dozens of sizes and variations to enable a relatively close fit for the vast majority of the population. For products such as furniture, this may be limited to three or four sizes, better known as small, medium and large. In fact, this was Herman Miller's solution to the chair fit problem - creating three different sizes allowed for fit of 95% of the population between the smallest 1st percent and highest 99 percent - a greater range then they had originally intended. During the design of the airport kiosk that we discussed in part 1, one of the early proposed solutions was to create a two-sided kiosk with a "low" and "high" screen positions that could comfortably suit a wide range of users.
Adjustability is really a special case of multiple sizes where the user (or an expert) modifies the fit at installation or during use. Most of us are familiar with adjusting the driver's seat in a car. These seats are not infinitely adjustable, but typically have three or more control points that can lead to a very wide range of positions, within the available space constraints. The downsides of adjustability are cost, reliability, and the extra work placed on the user to adjust the fit. Note, that many users may not always set the best fit for themselves.
Satisficing, is coming up with a single solution that fits the broadest range of users. In practice this tends to skew towards the smaller or shorter end of users because, larger users can always bend (although at 6' 4" I can say that's not always comfortable) and smaller users may have physical limitations due to age or disability that take priority (legal and otherwise). Most designs for public spaces will take this approach, as in elevators, water fountains and ATMs. For the kiosk, the best single solution is pictured below at a fixed height and distance that was manageable for a broad range of users:
Prototyping for Fit
Whether designing a single solution or multiple sizes, it is important to to follow a user-centered design process. There may be room in interface design for "genius-centered design", but there's no substitute for real-world measurement of physical fit. As in interaction design, prototyping can take many forms, depending on your goals and need for fidelity at each stage of the design process. For example, if the initial goal was simply to conduct a real-world test of key dimensions, then a simple sticker on a wall could serve as a "prototype" for display position. For more detailed issues, such as task-specific grips on a tool handle, foam mock-ups can be created and evaluated.
A typical UCD process for ergonomic fit would follow these steps, presented in an abbreviated form here:
- Define relevant populations (e.g. age range, nationality, sex)
- Define key dimensions or variable for fit consideration (e.g. height, reach, weight, etc)
- Determine boundary measures for each anthropometric dimension from reference data, from lower 5th to upper 95th percentile (keeping in mind that some dimensions, such as head clearance in a doorway, may be one-sided)
- Compare referenced dimensions with existing real-world products for reality check
- Apply dimensions to create mock-ups for initial, informal ergonomic feedback with users
- Refine design(s) to create foam or similar low-fidelity mock-ups for fit evaluation
- Continue to refine as needed/budgeted
In part 3 I'll get into specifics around actually measuring the "usability of fit", that is, the quantitative and qualitative measures to assess whether a design actually fits a range of users.
Part 2 Takeaways:
- Anthropometric variables such as height and reach should be considered as independent of each other. Therefore the more variables that you are designing for, the smaller that population that will fit across all of those dimensions.
- Human bodies do not have fixed aspect ratios like screens do, but it seems a little more than coincidental that widescreen displays became popular in synch with the growth in population obesity.
- Providing multiple sized designs or adjustability are pragmatic solutions when good fit is important, but in most cases, a single, satisficing solution is required.
- Use anthropometric data as a starting point to build mock-ups or prototypes, then evaluate fit - more to be discussed next time.