The survey collected data from a number of sources, including news accounts, courts, and corrections departments. Several States -- Colorado, New Jersey, New Mexico, North Carolina, Pennsylvania, and Tennessee -- have not sentenced anyone under their three-strikes statutes. Other States use them rarely. The exception is California, where some 15,000 people have been sentenced under its law. Reasons for the prosecutorial reluctance laws, which often mandate life in prison for serious, repeat offenders, may range from a desire for fairness to a habit of using other get-tough sentencing measures. Walter Dickey, author of the study and a law professor at the University of Wisconsin, suggests that prosecutors are using other tough sentencing laws and turning to three-strikes statutes as a threat in plea bargaining.