Math helps detect gang-related crime and better allocate police resources

February 14, 2013, Society for Industrial and Applied Mathematics
Left: Map of gang territories in the Hollenbeck area of Los Angeles. Right: LAPD FI card data showing average stop location of 748 individuals with social links of who was stopped with whom. Credit: Matt Valasik and Blake Hunter

(—Social groups in a population can lend important cues to law enforcement officials, consumer-based services and risk assessors. Social and geographical patterns that provide information about such communities or gangs have been a popular subject for mathematical modeling.

In a paper published last month in the SIAM Journal on Applied Mathematics, authors use police department records about individuals' social and geographical information to determine memberships.

Data on social interactions is particularly hard to come by, but in combination with , it can determine locations of specific groups in the population, such as gangs. For instance, if an individual's geographic location at a set of times is known, social interactions may be inferred by detecting people present at the same place at the same time. In this manner, hotspots at major gang locations can be determined.

In this paper, data is used from LAPD field interview cards, which document stops by of known or suspected gang members in the Hollenbeck area of Los Angeles. For each of the 748 gang members whose data is compiled, the average of all locations where they were stopped is determined, in addition to other individuals that may have been present at each stop. Due to the generally nonviolent nature of the stops, individuals stopped together are assumed to share a friendly or social connection.

A fully connected graph is constructed using this information with nodes that represent 748 individuals and an associated affinity matrix. An affinity matrix helps determine the similarity or likeness between two sets of objects or parameters. The matrix is a combination of social adjacency and an encoding function that depends on the average stop distances between individuals. In order to cluster the individuals, the authors use a technique called spectral clustering, a mathematical method used to identify groups of "similar behavior" in data. This identifies clusters in the graph based on the abovementioned social connections.

"One thing our study shows is that a clustering based purely on the locations where the people were stopped already provides clusters of over 50% purity, indicating the important role that geography plays," says lead author Yves van Gennip.

A greater amount of social data leads to improvements in clustering metrics. Since social data for such studies tends to be low, social information is extended past the current levels of sparsity by augmenting it with noise and available data on connections between members of the same gang. Such extensions of data can be particularly advantageous in tightly-controlled security settings, such as war zones. For instance, in the border regions of Afghanistan, taking into account familial, tribal and religious affiliations as well as social and economic concerns of individuals can help identify their support for insurgencies—active or passive. While intelligence information from the ground can be meager in such areas, meetings of groups of individuals can be easily observed. These methods can also be used to establish social, and hence, group interactions through geosocial information available from social media sites.

"The type of analysis undertaken in the paper can have practice applications for local law enforcement," explained one of the authors, George Tita. "While it certainly will not provide clues as to the particular individual that committed a gang-motivated crime, it can provide investigators a starting point with respect to the particular gang that might have been involved in the attack. Thus, the results of our analysis can provide a way for local police to allocate their scarce resources more strategically."

Including both social and geographic distance in models of gang violence such as this is seen to provide more comprehensive analysis – for example, in ecological models, even low levels of competition between gangs can produce sharp boundaries between gangs with a pattern of violence along borders. This "sociospatial" dimension can thus allow successful intervention to reduce gang violence. Targeted enforcement is seen not only to reduce crime in the area surrounding a gang, but also to diffuse through social networks, reducing violence among gang rivals.

"We are currently working on a much larger dataset that is similar to the one discussed in the paper," said Andrea Bertozzi, one of the authors. "The challenge is to develop algorithms that will use raw data from field interview cards from tens of thousands of people over several-years worth of events."

Explore further: Internet banging: Gangs use social media to trade insults, threats

More information:

Related Stories

Fighting violent gang crime with math

October 31, 2011

( -- UCLA mathematicians working with the Los Angeles Police Department to analyze crime patterns have designed a mathematical algorithm to identify street gangs involved in unsolved violent crimes. Their research ...

Gangs don't protect against crime

April 13, 2011

Gang members are twice as likely to be crime victims than non-gang members and are more frequently subject to simple assault, aggravated assault and drive by shootings, according to a recently study by the Crime Victims' ...

Recommended for you

Lifting barriers to citizenship for low-income immigrants

January 15, 2018

Taking the Oath of Allegiance at a naturalization ceremony is an emotional moment for many immigrants, and for good reason: it is the culmination of an often arduous process and many years of striving. Citizenship also opens ...


Adjust slider to filter visible comments by rank

Display comments: newest first

3 / 5 (4) Feb 14, 2013
Adhere to the model too closely, and you'll find that criminals use it to predict where a Cozzer won't be passing.
not rated yet Feb 14, 2013
Orient it to the actors and it will become apparent how important any particular individual is. Resources can then be allocated accordingly.

There is a ted talk on economics that illustrates this science of complexity:

Instead of who controls the world it may be titled who controls the streets.

1 / 5 (4) Feb 15, 2013
The last sentence makes no sense. It says they are attempting to get the program to use several years worth of field contact card information. But the info on those cards has a lifetime of months not years. Not to mention the entire effort is ridiculous.
1 / 5 (4) Feb 15, 2013
It has been my observation that whenever the cops identify an area where excessive crime activity occurs, they go in and bust it up scattering the criminals to less concentrated areas. Then they ask taxpayers for more resources to help them find the criminals.
not rated yet Feb 15, 2013
@dan, so how do you think they should solve the problem?
1 / 5 (5) Feb 15, 2013
Crime occurs where criminals are?
No way.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.