Restraining convicted criminals in prison, rather than releasing them back onto the streets, is a highly cost-effective way of cutting down on the number of murders, rapes, robberies, assaults and thefts.

• On average, it costs about $25,000 per year to keep a convicted criminal in prison.

• Various studies have indicated that keeping one prisoner incarcerated reduces the number of crimes committed anywhere from 12 to 21 each year.

• A Justice Department study estimated that tripling the prison population from 1975 to 1989 may have reduced violent crime by 10 to 15 percent below what it would otherwise have been.

That would amount to a conservatively estimated 390,000 murders, rapes, robberies and aggravated assaults prevented in 1989 alone. That's because so many of those in prison have committed multiple crimes. In fact,

• Some 94 percent of state prisoners in 1991 had committed a violent crime, or been incarcerated or on probation before their present incarceration.

• Of these prisoners, 45 percent had committed their latest crimes while free on probation or parole.

• While "supervised" on the streets, they inflicted at least 218,000 violent crimes -- including 13,200 murders.

• Violent crimes committed each year cost more than $400 billion in medical bills, lost days from work, lost quality of life -- and lost life.

The research shows it costs society at least twice as much to let a prisoner loose than to lock him up. Compared with the human and financial toll of revolving-door justice, prisons are a real bargain.

Source: Prof. John J. DiIulio, Jr. (Princeton), "Prisons Are a Bargain, By Any Measure," New York Times, January 16, 1996.