It isnt, and indeed it is rarely satisfied. The real question is how sensitive the resulting model may be to violations of this assumption. Greg Ridgway looked

It isnt, and indeed it is rarely satisfied. The real question is how sensitive the resulting model may be to violations of this assumption. Greg Ridgway looked at a natural experiment (proportion of drivers pulled over 1 hour before dawn, when the color of the driver is very hard to determine a priori, vs that proportion 1 hour after dawn, where it is easy) back when he was at RAND, and came to a similar conclusion. Ymmv. Obviously, "pulled over" is not the same as "shot" but the objective data is easier to come by. Especially if you don't tell the LAPD what specifically you're studying beforehand.