Thursday, July 21, 2016

A Fallacy of Composition

Peter Moskos is a sociologist by training, a professor at John Jay College of Criminal Justice, and a former Baltimore City police officer. In responding to the shooting of Philando Castile, he had this to say:
Honestly, in this shooting, with this cop, in this locale, I don't think there's a chance in hell Castile would have been shot had he been white. 
Nor did he think this was an entirely isolated incident; it reminded him of the (non-fatal) shooting of Levar Jones by Sean Groubert at a traffic stop in South Carolina. I had exactly the same reaction when I saw the Castile video, as did others. Even the Governor of Minnesota conceded that the shooting "probably would not have happened if he were white."

And yet, Moskos was unsurprised by Roland Fryer's recent claims of an absence of racial bias in police shootings:
I was not surprised by Fryer's conclusions... if one wishes to reduce police-involved shootings... there are good liberal reasons to de-emphasize the significance of race in policing.

Jonathan Ayers, Andrew Thomas, Diaz Zerifino, James Boyd, Bobby Canipe, Dylan Noble, Dillon Taylor, Michael Parker, Loren Simpson, Dion Damen, James Scott, Brandon Stanley, Daniel Shaver, and Gil Collar were all killed by police in questionable to bad circumstances... What they have in common is none were black and very few people seemed to know or care when they were killed. 
Moskos is not arguing here that the police can do no wrong; he is arguing instead that in the aggregate, whites and blacks are about equally likely to be victims of bad shootings. 

How can these two views be reconciled? If there is bias in individual incidents, ought it not to show up in aggregate data? Doesn't the congruence between the racial composition of arrestees nationwide and the racial composition of victims of police killings indicate an absence of bias, as Sendhil Mullainathan claimed a few months ago?

I have argued previously that it does not, because of systematic differences in the qualitative nature of encounters. If police initiate more encounters with blacks that are not objectively threatening (but may in some cases be subjectively perceived to be threatening) then parity in killings per encounter can indicate the presence rather than absence of bias. As Andrew Gelman put it at the time, it's all about the denominator

But Moskos offers another, quite different reason why bias in individual incidents might not be detected in aggregate data: large regional variations in the use of lethal force. 

To see the argument, consider a simple example of two cities that I'll call Eastville and Westchester. In each of the cities there are 500 police-citizen encounters annually, but the racial composition differs: 40% of Eastville encounters and 20% of Westchester encounters involve blacks. There are also large regional differences in the use of lethal force: in Eastville 1% of encounters result in a police killing while the corresponding percentage in Westchester is 5%. That's a total of 30 killings, 5 in one city and 25 in the other.

Now suppose that there is racial bias in police use of lethal force in both cities. In Eastville, 60% of those killed are black (instead of the 40% we would see in the absence of bias). And in Westchester the corresponding proportion is 24% (instead of the no-bias benchmark of 20%). Then we would see 3 blacks killed in one city and 6 in the other. That's a total of 9 black victims out of 30. The black share of those killed is 30%, which is precisely the black share of total encounters. Looking at the aggregate data, we see no bias. And yet, by construction, the rate of killing per encounter reflects bias in both cities. 

This is just a simple example to make a logical point. Does it have empirical relevance? Are regional variations in killings large enough to have such an effect? Here is Moskos again:
Last year in California, police shot and killed 188 people. That's a rate of 4.8 per million. New York, Michigan, and Pennsylvania collectively have 3.4 million more people than California (and 3.85 million more African Americans). In these three states, police shot and killed... 53 people. That's a rate of 1.2 per million. That's a big difference.

Were police in California able to lower their rate of lethal force to the level of New York, Michigan, and Pennsylvania... 139 fewer people would be killed by police. And this is just in California... If we could bring the national rate of people shot and killed by police (3 per million) down to the level found in, say, New York City... we'd reduce the total number of people killed by police 77 percent, from 990 to 231!
This is a staggeringly large effect. 

Additional evidence for large regional variations comes from a recent report by the Center for Policing Equity. The analysis there is based on data provided voluntarily by a dozen (unnamed) departments. Take a close look at Table 6 in that document, which reports use of force rates per thousand arrests. The medians for lethal force are 0.29 and 0.18 for blacks and whites respectively, but the largest recorded rates are much higher: 1.35 for blacks and 3.91 for whites. There is at least one law enforcement agency that is killing whites at a rate more than 20 times greater than that of the median agency.

On the reasons for these disparities, one can only speculate:
I really don't know what some departments and states are doing right and others wrong. But it's hard for me to believe that the residents of California are so much more violent and threatening to cops than the good people of New York or Pennsylvania. I suspect lower rates of lethal force has a lot to do with recruitment, training, verbal skills, deescalation techniques, not policing alone, and more restrictive gun laws. 
Moskos expands on these points in a recent conversation with Glenn Loury.

All of this must be interpreted with caution, since the information we have available is so patchy and deficient. As I wrote in a recent opinion piece with Willemien Kets, there is a desperate need for better data, collected and distributed in a comprehensive and uniform manner. Without this we are just groping in the dark.

Thursday, July 14, 2016

On Arrest Filters and Empirical Inferences

I've been thinking a bit more about Roland Fryer's working paper on police use of force, prompted by this thread by Europile and excellent posts by Michelle Phelps and Ezekeil Kweku.

The Europile thread contains a quick, precise, and insightful summary of the empirical exercise conducted by Fryer to look for racial bias in police shootings. There are two distinct pools of observations: an arrest pool and a shooting pool. The arrest pool is composed of "a random sample of police-civilian interactions from the Houston police department from arrests codes in which lethal force is more likely to be justified: attempted capital murder of a public safety officer, aggravated assault on a public safety officer, resisting arrest, evading arrest, and interfering in arrest." The shooting pool is a sample of interactions that resulted in the discharge of a firearm by an officer, also in Houston. 

Importantly, the latter pool is not a subset of the former, or even a subset of the set of arrests from which the former pool is drawn. Put another way, had the interactions in the shooting pool been resolved without incident, many of them would never have made it into the arrest pool. Think of the Castile traffic stop: had this resulted in a traffic violation or a warning or nothing at all, it would not have been recorded in arrest data of this kind.

The analysis in the paper is based on a comparison between the two pools. The arrest pool is 58% black while the shooting pool is 52% black, which is the basis for Fryer's claim that blacks are less likely to be shot by whites in the raw data. He understands, of course, that there may be differences in behavioral and contextual factors that make the black subset of the arrest pool different from the white, and attempts to correct for this using regression analysis. He reports that doing so "does not significantly alter the raw racial differences."

This analysis is useful, as far as it goes. But does this really imply that the video evidence that has animated the black lives matter movement is highly selective and deeply misleading, as initial reports on the paper suggested? 

Not at all. The protests are about the killing of innocents, not about the treatment of those whose actions would legitimately plant them in the serious arrest pool. What Fryer's paper suggests (if one takes the incident categorization by police at face value) is that at least in Houston, those who would assault or attempt to kill a public safety officer are treated in much the same way, regardless of race. 

But think of the cases that animate the protest movement, for instance the list of eleven compiled here. Families of six of the eleven have already received large settlements (without admission of fault). Six led to civil rights investigations by the justice department. With one or two possible exceptions, it doesn't appear to me that these interactions would have made it past Fryer's arrest filter had they been handled more professionally. 

The point is this: if there is little or no racial bias in the way police handle genuinely dangerous suspects, but there is bias that leads some mundane interactions to turn potentially deadly, then the kind of analysis conducted by Fryer would not be helpful in detecting it. Which in turn means that the breathless manner in which the paper was initially reported was really quite irresponsible. 

For this the author bears some responsibility, having inserted the following into his discussion of the Houston findings:
Given the stream of video "evidence", which many take to be indicative of structural racism in police departments across America, the ensuing and understandable outrage in black communities across America, and the results from our previous analysis of non-lethal uses of force, the results displayed in Table 5 are startling... Blacks are 23.8 percent less likely to be shot by police, relative to whites.
His claim that this was "the most surprising result of my career" was an invitation to misunderstand and misreport the findings, which are important but clearly limited in relevance and scope.


Update. If you follow the links at the start of this post, you'll see a case made that Fryer's own findings of bias in the use of non-lethal force suggest that the composition of the arrest pool will be altered by bias in the charging of innocents for resisting or evading arrest.

It occurred to me that the same data used to examine use of non-lethal force (from the citizen's perspective) could also be used to get an estimate of this effect. This is the Bureau of Justice Statistics Police-Public Contact Survey. If anyone had done already this please let me know, I'd be interested to see the findings.

Monday, July 11, 2016

Police Use of Force: Notes on a Study

A new empirical analysis of police use of force by Harvard economist Roland Fryer is attracting national attention. The paper deals with both lethal and non-lethal force, using a variety of different data sets, some public and some painstakingly assembled by the author and his team. Given the harrowing events of the past week, it's likely that his results on shootings will attract the most attention, but it's worth carefully considering both sets of findings.

Fryer provides evidence of significant racial disparities in the experience of non-lethal force at the hands of police, even in data that relies on self-reports by officers. Using official statistics from New York City’s Stop, Question and Frisk program, he finds that blacks and Latinos are more likely to be held, pushed, cuffed, sprayed or struck than whites who are stopped. This remains the case even after controlling for a broad range of demographic, behavioral, and environmental characteristics. And using data from a nationally representative sample of civilians, which does not rely on officer accounts, he finds evidence of even larger disparities in treatment.

But Fryer also reports an absence of racial bias in police shootings for a select group of jurisdictions. He recognizes that a proper analysis of police bias in the use of lethal force requires data not only on those incidents in which shootings occurred, but also those in which suspects were successfully pacified and disarmed. Data of this kind is extremely hard to come by, but he has managed to obtain incident reports on arrests in Houston that can be used for this purpose. 

The focus is on arrest categories that are more likely to involve incidents resulting in justified use of lethal force. It turns out that in this arrest data 58% of the population is black, while in the shooting data the corresponding share is 52%. This immediately implies that in the absence of controls for other features of the interaction, blacks in the arrest population are less likely to be shot than whites. He finds that controlling for other features of the interaction "does not significantly alter the raw racial differences." Here is how Fryer characterizes these findings:
Given the stream of video "evidence", which many take to be indicative of structural racism in police departments across America, the ensuing and understandable outrage in black communities across America, and the results from our previous analysis of non-lethal uses of force, the results displayed in Table 5 are startling... Blacks are 23.8 percent less likely to be shot by police, relative to whites.
He describes this as "the most surprising result of my career."

While it is entirely possible that the Houston Police Department doesn't exhibit systematic racial bias in the use of lethal force, I'm not sure such an emphatic conclusion is warranted. A close look at the arrest data (Table 1D) alongside the shooting data (Table 1C, column 2) reveals a number of puzzles that should be a cause for concern. In the arrest data only 5% of suspects were armed, and yet 56% of suspects "attacked or drew weapon." This would suggest that over half of suspects attacked without a weapon (firearms, knives and vehicles are all classified as weapons). Moreover, there are large differences across groups in behavior: two-thirds of whites and one-half of blacks attacked, a difference that is statistically significant (the reported p-value is 0.006).  

What this means is that the pool of black arrestees and the pool of white arrestees are systematically different, at least as far as behavior is concerned. So the raw data comparison described as startling in the quote above is not really valid. (I made a similar point in response to a piece by Sendhil Mullainathan a few months ago). Still, Fryer controls for these differences in behavioral and contextual characteristics and finds that the basic picture doesn't change. This has to be taken seriously. The key question, to my mind, is whether these controls are adequate. 

I personally would be more convinced if the arrestee pool looked more like the shooting victim pool. For instance, 18% of arrestees, but only 4% of shooting victims are female. I suspect that many of the interactions in the arrestee pool are not threatening, even from the subjective perspective of the officers involved. And others are so obviously threatening---for instance those involving suicide-by-cop---that no discretion or judgement is really necessary. Pruning these from the data might give us a clearer picture of bias in the use of discretionary lethal force. 

Despite these concerns, I think that there is a case to be made that there is no systematic bias against blacks in the lethal use of force within the Houston Police Department. What one ought not to conclude, however, is that this applies nationally. The analysis of other jurisdictions considered in the paper is restricted to encounters in which shootings actually occurred, and cannot therefore be used to answer the same kinds of questions that the Houston data allows. 

One last point about shootings: I'm not sure why there are quotation marks around the word "evidence" in the above quote. Video evidence, for all its flaws, is still very powerful evidence. It was video evidence that led to the indictment of Micheal Slager on murder charges, and the conviction of Sean Groubert for assault and battery. It is selective and cannot establish the presence of racial bias in individual cases, but surely it can't be dismissed out of hand.

Finally, consider Fryer's analysis of non-lethal force, which is consistent with earlier findings. Aside from being fundamentally unjust, disparities in the use of non-lethal force have some really important implications for crime rates. The harassment of entire groups based on racial or ethnic identity is a major obstacle to witness cooperation in serious cases, including homicide. In fact, given the importance of corroboration, a belief that other witnesses will not step forward can be self-fulfilling.

With witnesses routinely unwilling to come forward in some neighborhoods, people can be killed with near impunity. And this significantly increases the incentives to kill preemptively, in a climate of reciprocal fear. Low clearance rates for homicide are directly responsible for high rates of killing, and both of these are held in place by distrust of the criminal justice system by potential witnesses. The excessive and discriminatory use of non-lethal force by police thus ends up having indirect lethal effects.

Thursday, July 07, 2016

Deadly Stereotypes

This video is hard to watch but important to think about and learn from:

Here's what appears to have happened. At around 9pm on July 6, Philando Castile was stopped for a broken taillight while driving in Falcon Heights, Minnesota. He was accompanied by his girlfriend, Lavisha Reynolds, and her young daughter. On being asked for his license and registration, Castile informed the officer that he had a firearm in the vehicle, and a concealed carry permit. He then reached for his wallet and was fatally shot. The video above captures the aftermath of the shooting, and was streamed live to a facebook account by Reynolds. 

The incident immediately brought to mind the shooting of Levar Jones by Sean Groubert in September 2014, which was captured on the officer's dashcam video. Again, there was a traffic stop, a request for documents, and multiple shots fired as Jones reached for his wallet:

Jones was hit but survived the shooting, and Groubert would later plead guilty to assault and battery.

What ties these incidents together is that they seem to have been motivated primarily by fear rather than anger or malice. Moreover, this fear turned out to have been unwarranted: neither Jones nor Castile posed an objective threat to the respective officers. The same was true of Amadou Diallo back in 1999, and in the more recent cases of Tamir Rice and John Crawford.

Whether or not the fear was reasonable under the individual circumstances of each case is harder to ascertain, and there is usually enough doubt to preclude criminal prosecution. Nevertheless, there are rare instances in which the unreasonableness of the fear is recognized: Groubert's employment with the South Carolina Department of Public Safety was terminated on the explicit grounds that he "reacted to a perceived threat where there was none."

A question of great moral and social importance is whether or not such fear is driven, in part, by exaggerated stereotypes of black male violence held by some subset of officers. The anecdotal evidence certainly suggests that such stereotypes matter on average, even if they are not implicated in every case. There is also some evidence of implicit bias from video game simulations.

Further evidence can be found in a dataset assembled by The Guardian. According to this source, there were a total of 1,145 police killings in 2015 alone, about half of which involved suspects armed with a gun. A further 13% of those killed were armed with a knife. There is no question, therefore that police officers often face armed and dangerous suspects. However, 18% of whites killed by police in 2015 were unarmed while 52% had a gun; the corresponding figures for blacks were 25% and 46%. This suggests that within the set of encounters that result in police killings, those involving black suspects are less objectively threatening to the officers involved. One possible explanation is that any given encounter is more likely to be perceived by the officer as threatening when the suspect happens to be black.

In the Guardian data, slightly more than half of those killed by police were white, 27% were black, and 17% Latino. The proportion of those killed who were black is roughly the same as the proportion of total arrestees who are black, which has led some to argue that "removing police racial bias will have little effect on the killing rate." But this claim depends on the questionable assumption that encounters involving black citizens are as likely to be objectively threatening to officers and encounters with white citizens. As I have argued previously, there are reasons to believe that they are not.

The health of our society depends on an effective and trusted criminal justice system. In fact, the system cannot be effective if it isn't trusted. Distrust makes witnesses to crimes unwilling to come forward and depresses clearance rates. This allows serious crimes, including homicide, to be committed with impunity. Fear of homicide victimization raises incentives for preemptive killing, resulting in epidemics of violence. At the heart of it all are stereotypes, affecting interactions between victims and offenders, parties to disputes, prosecutors and witnesses, and officers and suspects. And the very same stereotypes also affect the urgency and concern with which the general public views mass incarceration

What can be done? The screening and training of officers has got to take into account the possibility that stereotypes can be deadly. Psychologists have found that exposure to counterstereotypical exemplars can reduce implicit bias, and residency requirements can serve as a screening device. Finally, the construction of a complete and consistent national database of incidents remains imperative. Public action requires broad engagement with the issue and some agreement on the nature of the problem, and this will not be possible while arguments continue to rely on anecdotal and indirect evidence. Such evidence is too quickly dismissed by skeptics and too easily filtered by stereotypes, no matter how shocking and heartbreaking and deeply persuasive a sympathetic observer finds it to be.