The ‘Average’ Myth
In 1950 they decided to find out. Researchers at Wright Air Force Base in Ohio measured over 4,000 pilots on 140 dimensions of size, including average torso length, arm length, crotch height and even thumb length.
[Lt Gilbert] Daniels selected ten physical dimensions that he thought would be most important for cockpit design. Using the data from the 4,063 pilots who had been measured, Daniels defined someone as average if their measurements fell within the middle 30% of the range for each dimension.
He then compared each individual pilot to the average he had calculated. Most of his colleagues expected the vast majority of pilots to be within the average range for over half the dimensions. But in fact Daniels analysis discovered none of the 4,063 pilots measured managed to fit within the average range of all ten dimensions. Even when he selected only three dimensions fewer than 3.5% of pilots were within the average size for all three dimensions.
via Conversion Uplift
The article does a good job explaining the applicability of "average" in user-experience design, but I’ll add one more trick that I’ve always used: histograms.
If you’re designing a new shopping cart experience, your research may find the average number of products in the cart is 50. But, 90% of those carts may have only one item, while the other 10% have hundreds. Designing the cart to maximize the experience of someone with 50 items won’t help anyone.
Or: statistically, the average human being has fewer than two arms.