A new study on police force found no bias against black civilians in police shootings in 10 cities and counties, including Houston. It did find bias against blacks in every other type of force, like the use of hands or batons. The study provoked debate after it was posted on Monday, mostly about the volume of police encounters and the scope of the data it used. Below, the author of the study, Roland G. Fryer Jr., a professor of economics at Harvard, answered questions from readers.

What about the chance of a police encounter?

Mr. Fryer’s study looked only at what happens once the police have stopped civilians, not at the chances of being stopped in the first place. Many readers questioned whether that was the right denominator. Other research has shown that blacks are more likely to be stopped by the police.

Lee Buttala from Ashley Falls, Mass., asked, “Is it possible that the statistics on shooting are misleading because the police are less likely to stop white people generally.”

Wendy Maland from Chicago put it this way: “The question isn’t — once police identify you as a potential criminal, how are you treated? — the question is — who is being treated as a criminal?”

Mr. Fryer: I agree that blacks are more likely to be stopped, more likely to be harassed and more likely to be arrested. Ideally we would be able to set up an experiment to understand potential differences before an encounter. Unfortunately, that would require us to randomly assign civilian race in encounters of police, which isn’t possible! Given this limitation, we need to make the best out of available data. There are two important things I want to note: 1. The types of encounters that lead to police shootings in the videos that we have all seen are not the most common that actually occur in the data. In Houston, for instance, most of the officer-involved shootings come from calls for service resulting from burglaries or violent crimes, not from chasing down people with broken taillights. 2. I totally agree that deciding who to stop in a police stop is highly problematic and there certainly may be racial bias in that decision. So let’s think about the officer-involved shootings in which there’s a robbery in progress or a violent crime. Those are less likely to be plagued by selection bias in the decision of who to harass or stop. Analyzing only those cases yields similar results. Moreover, when we analyze only cases in which the officer-involved shooting began with a routine stop or a traffic stop, we do not find bias. But these results are susceptible to your point that there’s more traffic stops of blacks.

Why the focus on Houston?