The Washington Post has numbers on the much higher rate at which blacks are stopped and/or searched in Ferguson vs. whites. By itself, while that is a useful pointer to a discrimination issue, someone might argue that blacks in the area commit more crimes per capita and thus warrant more stops.
There is one bit of data in the Post's numbers that can be used to partially address this. The data says that blacks have their cars searched much more frequently than whites. Blacks have their cars searched 12.13% of stops while whites have their cars searched only 6.85% of stops. But this understates the disparity, since blacks are stopped at a higher rates than whites. Taking the disparity in stops in to account, blacks are searched at a rate 6 times higher than for whites.
The interesting part is in the data on contraband hit rate, ie the rate that searches uncover something illicit. The contraband hit rate for white car searches is 57% higher than for black car searches. In other words, it is more likely searches of white cars will yield something illegal. Which tends to undercut the argument that the greater rate of black car searches is somehow justified.
By the way, I want to highlight one other figure. Black cars are stopped at about a 6x higher rate for "equipment" deficiencies than whites. Nitpicky regulations on car conditions (in Arizona your licence plate frame cannot cover any part of the word "Arizona" on the licence plate) are the great bugaboo of the poor and a nearly unlimited warrant for the police to stop minorities. Mexicans here in Phoenix will tell me "woe to the Mexican who drives around here with a broken tail light -- he will be pulled over 3 times a day to have his immigration status checked". In Phoenix, at least, stops for equipment issues are roughly the equivalent of pulling someone over for "driving while brown." Even beyond the open-ended warrant these silly violations give the police, the fines and court costs create meaningful indebtedness problems for the poor which are hard to overcome.
(As a mostly irrelevant aside, I worked essentially in one corner of Ferguson in the Emerson Electric headquarters for a couple of years. Like many of the inner ring of suburbs in St Louis, this is not a wildly prosperous area but it also is not Somalia. Driving at night I was much more nervous in the neighborhoods both due south and due East of Ferguson).
Update: Via Zero Hedge