Design for Ranges Not Averages
Therefore, we often talk about designing to accommodate ”5th percentile female users through to 95th percentile male users”. This means that we are taking the 5th percentile female value in a particular characteristic as our ‘lowest’ design criteria and the 95th percentile male value as the upper bounding case.
In the example of stature (height), assuming a UK general working age population, this would mean that the design needs to accommodate users between approximately 1514mm and 1869mm in height.
The design range that we choose (or have a requirement to accommodate) relates to the concept of exclusion.
If we have a single-ended design constraint (in the example of the doorway, for example), designing to the 95th percentile stature will still mean that the door may be unsuitable or uncomfortable for the tallest 5% of the male population.
In these cases, it may be necessary to consider the 99th percentile as the upper bound design target. This will depend on the importance of the criteria. For example, if the doorway is to be used in an emergency such as a fire, it would not be acceptable to make the door difficult to use for the tallest 5% of the population. The same is true for machinery barriers and guards.
By designing for a range of human capabilities, designers can create products and environments that are more widely accessible and usable, which can ultimately lead to increased satisfaction and usability for all users, including the average user.