Philadelphia should take a page from Chicago’s playbook. For several years, Chicago has opened its crime gun trace data to the public. Working with the University of Chicago, police have analyzed the data and learned that 95 percent of crime guns in Chicago are traced back to someone other than the original purchaser, and that 91 percent of those guns wind up in the hands of felons. That evidence of a substantial illegal gun market helps Chicago educate the public and make the case to lawmakers and judges to crack down on repeat gun offenders. Also working with academics, Chicago has written an algorithm to help predict retaliation crimes.