[im3w1l]

It is "adjusted for age, gender, race, and educational attainment."

84% where white.

59% of Americans identify as middle class, upper-middle or upperclass. So saying that the study used mostly upper- and middle-class doesn't really tell us anything.

[sukilot]

Since the wealthier people call themselves middle class, 59% is approximately the 95th to 36th percentiles.