The Hound of the Data Points

As the train raced, Rossmo began jotting an equation on a napkin. The equation was too long, so he continued it on a second napkin. After a few minor changes in succeeding months, it ended up looking onClick="window.open('','popup1','height=104,width=500,scrollbars=no,resize=no')" target="popup1" class="sidebar">like this.

It seems impossibly complicated, but the equation is really expressing a couple of fairly basic principles. Along with a modification of the Brantinghams' crime-pattern theory, Rossmo incorporated something called the "least effort" principle, which says that human beings don't make a move before performing a little mental cost-benefit analysis. (You're not going to go across town to buy milk, for example, but you might to buy snow tires.) Another key principle was "routine-activity theory," which holds that crimes happen at the junction of familiarity and opportunity. For a crime to occur, "a motivated offender must encounter a victim in the absence of protection" (such as police, security or interfering witnesses). A criminal's behavior is the product of the often mundane decisions he has previously made -- and each of those was determined by where he happened to be when he made them.



Later, Rossmo tucked the napkins into his pocket. When he returned home he turned the equation into an algorithm, which would ultimately become a computer program -- the same Rigel (pronounced rye-gel and named after a star in the constellation Orion, "the hunter") loaded into this Dell Pentium 4 here in D.C.



Rossmo swivels his chair and, to demonstrate how Rigel works, pulls up on the screen a case from his files. It's the story of Lee Marvin Payne, a serial sexual-assault felon who prowled Mississauga, Ontario, in the 1990s.



The sites where Payne attacked his victims appear as red points, superimposed on a map of the
city. (Rigel is built like a sandwich, with code written in Vancouver
by employees of Rossmo's company -- Environmental Criminology Research -- as the base, then an interface, then Microsoft's routing software, Mappoint.)



Rigel chews on the data points. What it's doing is applying Rossmo's algorithm to calculate, for each of the 40,000 tiny squares on the grid, a "hit score," or probability that that very square is the rapist's operational base, or "anchor point." Rigel performs 440,000 calculations, and when it is done 10 seconds later, a graphic blooms on the screen. The profile looks a bit like a relief map. Toward one corner of the map, a fiery amoeba shape pops out, about 5 percent the size of the original search area -- the "peak profile" area Rossmo has asked the computer to show him. "In this case the offender lived here," he says, pointing to a spot on the yellow shoulder of the red "hot zone" in the middle. That's a hit score of about 2 percent -- meaning the police would have to search an area of only a few city blocks before they knocked on Lee Marvin Payne's door.



Rossmo punches in another command, and a sort of double-peaked volcanic cone rises from the plane. This is the "jeopardy surface," a kind of topographical probability map, with the hot zone shown as the peak of the volcano.