At around the 12 minute mark of her talk, Kat Holmes brings up how cockpits of fighter jets were initially designed by measuring a large sampling of pilots and using the average of those measurements to dictate positions and dimensions of spaces in the plane.

In this way, the military was trying to design for everyone, but what Holmes explains is, they were really designing for no one. After many crashes due to design errors, a junior designer took the average pilot measurements and applied them to a large sampling of pilots, finding that none of the dataset fit ALL of the measurements exactly. Some pilots had perfectly average arm and torso lengths, but below average leg length. All of the things that make us unique as people were throwing a wrench in all the well laid plans of the military.

By discarding the average as their reference standard, the air force initiated a quantum leap in its design philosophy, centered on a new guiding principle: individual fit. Rather than fitting the individual to the system, the military began fitting the system to the individual. In short order, the air force demanded that all cockpits needed to fit pilots whose measurements fell within the 5-per-cent to 95-per-cent range on each dimension.

Innovations in adjustable chin straps, seat placement, things that, any time you get in a car, you’re benefiting from the design of this process. As UX designers we need to take this same approach when we apply our user personas to products we’re working on. Keep in mind that no matter how painstakingly crafted your personas are, they are definitely not exactly any single user in your dataset, so we must design accordingly for a level of adjustability.