Nobody is average height and average weight and average arm length and…. Choose enough parameters, and nobody is average in all of them. “One size fits all” actually fits no-one.
Rose provides examples of where the assumption that there is an “average person” can go badly awry (for example, fitting variable pilots into standard cockpits), and where the idea came from historically. There are competing historical camps: one has the average as a measure of “perfection” with outliers being misfits; the other has that deviations from average are correlated, so someone above average height, say, will also be above average everything else. Both camps are wrong: everyone is an outlier in multiple dimensions, and deviations aren’t well correlated.
Rose goes on to describe the perils of standardisation, and how variation needs to be accommodated, and how it can be an advantage: different people are good at different things.
A readable and informative little book, this should be read by everyone responsible for evaluating or designing things for others, from job interviews to education, via cockpits.