In 1926, US military leaders were designing an aeroplane cockpit for the first time. The bodies of hundreds of pilots were tape-measured, and the average of those measurements informed designs for the size and shape of the seat and the height of the windshield. For decades, that standard design endured.
Then, in 1950, it was time for an update. Gilbert S. Daniels, a 23-year-old lieutenant, was part of the core redesign team. This time, the team measured over 4,000 pilots, studying 140 separate dimensions: Thumb length. Crotch height. Span from eye to ear. The idea was that with more precise specifications, a more ergonomic cockpit could be created, which would lead to better pilot performance and fewer crashes. Or so it was thought. Daniels was sceptical. Were the “average” dimensions really the key consideration? How many pilots were average?
Daniels combed through the data. He analysed 10 physical dimensions in particular, which included height and sleeve length, that were thought to be critical for designing cockpits. For each dimension, he computed the average. For height, it was 5’9”. Then he broadened the average to include everyone who was in the middle 30% of the distribution. So, with height, the 30% middle range included all pilots between 5’7” and 5’11”. Daniels asked himself, how many pilots were in the middle range on all 10 dimensions? Here’s what Daniels found, according to Todd Rose, who tells the story in his fascinating book The End of Average: even Daniels was stunned when he tabulated the actual number.
Zero.
Out of 4,063 pilots, not a single airman fit within the average range on all ten dimensions. One pilot might have a longer-than-average arm length, but a shorter-than-average leg length. Another pilot might have a big chest but small hips. Even more astonishing, Daniels discovered that if you picked out just three of the ten dimensions of size—say, neck circumference, thigh circumference, and wrist circumference—less than 3.5 percent of pilots would be average-sized on all three dimensions. Daniels’s findings were clear and incontrovertible. There was no such thing as an average pilot.
If you’ve designed a cockpit to fit the average pilot, you’ve actually designed it to fit no one. The air force’s leaders, to their credit, took this finding seriously. They abandoned the idea of tailoring the cockpit to an “average pilot.” Instead, they asked airplane manufacturers to create an adjustable environment, one that could accommodate pilots whose measurements spanned almost the full spectrum of possibilities: from the 5th to the 95th percentile on each dimension.
The manufacturers squawked. Such customisation would be too expensive! Too slow! Eventually, though, they did what most business leaders do when confronted with losing gigantic customer orders: They caved. And they figured it out. Suddenly, everything could be tweaked. Adjustable foot pedals. Adjustable seats. Adjustable helmet straps. All could be tailored to the individual pilot. As a result, pilot performance soared in the years afterwards.
This study is also the reason every seat in cars today can be adjusted along a number of dimensions.
Averages are great for monitoring but terrible for diagnosis. Averages can tell us: Something’s wrong. But they’re unlikely to tell us what’s wrong or how to fix it.
If averages are useless, then what should you be looking at?
Exceptions.
Take the example of a call centre where the average caller signs up 15 new clients, but there is one individual who is signing up 45 new clients; you want to know the source of that bright spot. What is that individual doing differently? What is the process that he/she follows? The bright spots tell you what works, where averages will fail to tell you anything meaningful.