If we design for the “average person”, we’ve covered our bases right? The average person would theoretically have average-sized arms, hands, legs, and feet, so one might think that we should be able to accommodate most people by designing for the average.
Unfortunately it’s not that simple. Design for average typically leaves those at the extremes in awkward postures. (Think about a really tall person in a bathtub, or a really small person typing at the “standard” desk height.) When our team at Taylor’d Ergonomics is asked, “How high and how low should my height-adjustable table go,” we typically respond with a range that would allow a small female and a tall male to work at a comfortable height. However, even by designing for 95% of the working population, we are actually designing out 5% of employees!
Anthropometric charts report dimensions for different “percentiles”. A 95th percentile person is bigger than 95% of the population, and smaller than 5% of the population. The charts tell us that a “5th percentile” (small) female is 152 cm tall (just under 5′), and should have an elbow height of 94.5 cm (about 37″). She should have a hip width of 31 cm (about 12″). Many of us know 5′ tall females with hips plenty wider than 12″.
Real people rarely come in “chart” dimensions. A person who is 5th percentile in height is not 5th percentile in all of his/her dimensions; this is an important concept to understand in the design world. You like a size “medium” shirt for width, but the sleeves might always be too short.
When we design using charts, we assume that, if we design a product with features that adjust for 95% of the population, then 95% of the customers will be satisfied with the result. The problem is that each feature “designs out” a different 5% of the population. Consider the design of office chairs. If we design the seat pan height to accommodate 95% of the population, then we know that 2 or 3 in 100 people will not be able to touch the floor no matter how low they set the seat, and 2 or 3 in 100 people will have their knees higher than their hips with the seat at its highest setting. If we also design the seat pan width to be wide enough for 95% of the population to fit between the armrests, then we know that 5% of the population will be “squeezed in”. The 5% that we “designed out” with seat height is not necessarily the same 5% that we “designed out” with the seat width. Our chair, designed for 5%, may only accommodate 90% of the population.
Some companies are pursuing “virtual reality” design processes. Ultimately, we should be able to anticipate all sorts of ergonomics problems before they become “set in stone”. In the meantime, we are pleased when our clients ask us for input into working heights and reaches on jobs that are in the design stage! “Chart” ergo design guidelines allow us to ensure that the heights, reaches, and clearances are reasonable for most workers. Our biomechanical models allow us to evaluate force demands. For now, trying to accommodate 95% of the population is the most reasonable objective for most companies. For training on how to design for “more than average”, please join us for our one-day “Ergo Design” course on May 30th.