A Mathematical Method to Locate Killers

ByABC News
October 7, 2002, 12:15 PM

Oct. 8 -- If the shootings of eight people this past week in Maryland and nearby areas are the work of a single suspect, then mapping and analyzing the locations of each crime can help investigators locate the killer's base.

One way to do this is using a recently developed software program called geographic profiling.

"The coordinates for each crime location is fed into the system," explains Kim Rossmo, director of research at the Police Foundation and creator of the geographic profiling system. "Then the resulting geometric pattern is interpreted by the algorithm. The outcome shows the probability of the residence of the perpetrator."

The program is based on a complex algorithm that Rossmo formulated using a collection of general rules applying to serial offenders. A series of crimes such as the recent shootings may seem random, but as Rossmo argues, they usually follow a pattern.

"There are stranger-to-stranger crimes but they're not random crimes in terms of where a person chooses to hunt or commit the crimes."

The Nearness Principle

A key rule, for example, is the "nearness principle." A large body of forensic research has shown that most individuals tend to commit crimes fairly close to home.

"Criminals don't function that differently from ordinary citizens when they do things such as shopping," says Rossmo. "They usually remain within a limited range. They also don't function that differently from animals, who tend to forage within a limited range from their base."

The extent of an offender's range depends on his or her preferred mode of transportation. A sniper operating from a vehicle would have more range, for example, than one using a bicycle or one who travels on foot. The geographic profile system incorporates all methods of transportation available to an offender, including buses, subways and cars when calculating a likely base.

There are some exceptions to the nearness principle. Rossmo says that older offenders often travel farther than younger ones and whites appear willing to travel further than blacks to commit crimes. But as a general rule, Rossmo says, people "don't go any farther than we have to to accomplish our goals."