People vs. Dimension percentiles - Pro version
PeopleSize Professional distinguishes between 'Dimension Percentiles' and 'Design Percentiles'.
This is an important distinction, because a series of (say) 95th percentile dimensions excludes more than 5% of the user group. See the Why Design Percentiles are not Dimension Percentiles topic for an explanation of this.
Often it is the intention to accommodate this 95% of people, so designs are unwittingly under-specified. This is a widespread problem.
The real overall proportion of people is hard to calculate. Some dimensions are closely related, so that people who are big in one dimension tend to be big in the other as well - for example, stature and eye height. In that case, you tend to exclude the SAME PEOPLE each time, so that the second dimension does not add much to the total number of exclusions. Some other dimensions vary independently, for example elbow height and nose breadth, so in that case you tend to exclude DIFFERENT PEOPLE each time, and the exclusions accumulate.
With several dimensions, each varying to a different degree with all the others, the predictive calculations become so difficult that a special method is used, called Monte-Carlo Modelling.
The Auto facility uses this process to calculate the Dimension Percentiles required to achieve a specified Design Percentile. This is the best way to decide what Dimension Percentile to set for your design.
We advise you always to use the Design Percentile facility, to make sure that your design fits the proportion of people that you intend.
Selecting dimensions for Design Percentile calculations