Suspects behavior towards the officer, suspects opinion of police based on past experience, etc? There are so many variables many of which are subjective to the people involved AND those doing the study to maintain enough control factors.

If a black person and white person separately encounter officers for the same reason- 1 refuses to cooperate, is agitated &/or aggressive to officers during a domestic disturbance call which are the most dangerous for officers and makes a quick movement reaching into their jacket. 2nd person also in their home from same disturbance call, is calm and cooperates, answers questions, keeps hands in full view and maybe only has gym shorts on so unlikely to have a weapon concealed. Can those scenarios be considered similar enough to show the outcome was racially motivated?