Caving in on civil rights
The legal systems of western nations such as the United States are fundamentally structured around the basic notion that people have the right to life, liberty, and the pursuit of property. More specifically, the U.S. Constitution's Fourth Amendment explicitly declares that people have a right "to be secure … against unreasonable searches and seizures, shall not be violated … but upon probable cause." Because predictive policing methods have become more popular, a consternation has arisen among informed U.S. citizens. People are concerned that predictive policing represents an encroachment on their Fourth Amendment rights.To see how this rights violation could occur, imagine that you've developed a predictive model that estimates a car theft will occur on the afternoon of January 15 at the corner of Apple Street and Winslow Boulevard. Because your predictions have proven accurate in the past, the agency dispatches Officer Bob to police the area at said time and day. While out policing the area, Officer Bob sees and recognizes Citizen Daniel. Officer Bob had arrested Citizen Daniel five years earlier for burglary charges. Officer Bob testified against Citizen Daniel and knows that he was subsequently convicted. Citizen Daniel is also a racial minority, and Officer Bob finds himself being suspicious on that basis alone (known as racial profiling, this is illegal, but it happens all the time).
Officer Bob, on said street corner, has a predictive report that says a theft crime is about to occur, and he's in the presence of a man of a racial minority whom he knows has a history of committing theft crimes. Officer Bob decides that the predictive report, combined with what he knows about Citizen Daniel, is enough to justify probable cause, so he performs search-and-seizure on Daniel's person.
The conundrum arises when one considers whether a predictive report combined with knowledge of past criminal activity is sufficient to support probable cause. Even if the predictive report were guaranteed to be accurate — which it's not — couldn't this decision to search Citizen Daniel just be mostly racial profiling on the part of Officer Bob? What if Officer Bob is just using the predictive report as a justification so that he can harass and degrade Citizen Daniel because Daniel is a minority and Officer Bob hates minorities? In that case, Officer Bob would certainly be violating Daniel's Fourth Amendment rights. But because Officer Bob has the predictive policing report, who's to say why Officer Bob acts in the way that he does? Maybe he acts in good faith — but maybe not.
Predictive policing practices open a gray area in which officers can abuse power and violate civil rights without being held liable. A significant portion of the U.S. population is against the use of predictive policing measures for this reason, but the approach has technical problems as well.
Taking on technical limitations
Data science for crime analysis is a special breed, and as such, it's subject to certain problems that may not generally be an issue in other domains of application. In law enforcement, criminal perpetrators are acting according to their own intellects and free will. A brief analogy is the best way to describe the problem.Imagine that you build a crime travel demand model. Based on the zone of origination, this model predicts that Criminal Carl will almost certainly travel on Ventura Avenue or Central Road when he goes to pick up his next shipment of drugs. In fact, the model predicts these same two routes for all drug criminals who depart from the same zone of origination as Criminal Carl.
Based on this prediction, the agency sets up two units, one on Ventura Avenue and one on Central Road, in the hope of catching Criminal Carl after the buy. Criminal Carl, of course, doesn't know about all these plans. He and his buddy Steve travel Ventura Avenue, purchase their drugs, and then return back along the same route. It's nighttime, so Steve isn't so worried about wearing his seatbelt since he figures no one could see that anyway. As Criminal Carl and Steve make their way back, Officer Irene begins to tail them and look for a reason to pull them over; Steve's seatbelt infraction is just cause. When Officer Irene talks to the men, she can tell they're high, so she has probable cause to search the vehicle. Criminal Carl and Steve go to jail on drug charges, and when they're released, they tell their criminal friends all the details about what happened.
The agency uses this model to catch six more drug offenders in rapid time, on either Ventura Avenue or Central Road. Each time they make an arrest, the offenders go out and tell their criminal associates the details of how they got caught. After six busts on these two roads within a relatively short period, local drug criminals begin to catch on to the fact that these roads are being watched. After word is out about this fact, no criminal will take these streets any more. The criminals change their patterns in random ways to avert police, thus making your predictive model obsolete.
This kind of common pattern makes it ineffective to use predictive models to reduce crime rates. After criminals deduce the factors that put them at risk, they avoid those factors and randomly assume a different approach so that they can perpetrate their crimes without being caught. Most of the time, agencies continually have to change their analysis strategies to try to keep up with the criminals, but the criminals are almost always one step ahead.
This is a more severe version of an issue that arises in many applications, whenever the underlying process is subject to change without notice. Models must always be kept up to date.