How Did the DNA Field Experiment Work?

From November 2005 to July 2007, each of the five jurisdictions collected biological samples — evidence thought to contain human cells in the form of hair, tissue, bones, teeth, blood or other bodily fluids — from the first 500 property crime scenes in which this evidence was found. Researchers then randomly divided the cases into two equal groups: the “treatment” cases and the “control” cases. In the treatment group, cases were investigated in the traditional way and the evidence was analyzed for DNA. In the control group, cases were investigated in the traditional way and evidence was not analyzed for DNA for at least two months.