The purpose of his study was to determine whether electronic monitoring was effective or not effective. Efficacy was measured by outcomes. He wanted to determine whether there was recidivism—whether they successfully completed the program, committed another crime, or committed violations of their terms of probation.
He did a lot of statistical analysis that I can't, unfortunately, quote off the top of my head, but the end result was, he said, that there was a dramatic benefit, when electronic monitoring and specifically GPS was used, in reducing recidivism.