The two types of EM equipment are continuous signaling and programmed contact; some types combine features of these two types. Many corrections agencies have EM programs, although the use of EM has not increased as fast as some had hoped or expected. Some programs are limited to particular groups of offenders such as drunk drivers, while others focus on a particular criminal justice phase such as probation. Many studies have focused on effectiveness; few have used experimental designs. Commentaries on EM have focused on the potential for net widening, unrealistic expectations, hidden costs, and the potential of EM to replace prisons. State laws mention EM in varying ways. The American Correctional Association has published program standards for agencies that only or primarily provide EM services. Corrections textbooks contain discussions of EM devices, although they usually do not present new findings or original studies. Some media articles have explained programs as human interest studies; others have focused on problems or have noted that EM has not grown as fast as originally hoped. Reference notes