( CNN ) -- It rarely makes sense to draw big conclusions or make public policy on the basis of anecdotes . But the plural of `` anecdote '' is data , and sometimes one-off events are useful in crystallizing lessons to guide policymakers and inform the public . So it was with the Pittsburgh-area rampage this week in which a teenager bearing two kitchen knives is accused of injuring 21 high school classmates and a security guard -- but none of them were killed . It 's hard to imagine an anecdote that better illustrates what decades of data show : that for purposes of life and death , the weapon matters . It 's called `` the instrumentality effect , '' and we owe the original scholarly findings ( more than four decades old at this point ) to the eminent University of California criminologist Franklin Zimring . Others , including one of us ( Cook ) , have validated and built upon his insights . The idea that the weapon matters emerges in studies of robberies and assaults . When committed with a gun , these crimes are far more likely to result in the victim 's death than are similar violent crimes committed without a gun . For example , the likelihood that a victim will die when robbed by a firearm-wielding attacker is three times as high as when the victim faces an attacker bearing a knife and 10 times as high as when the attacker has another type of weapon . For victims injured in an assault , the likelihood of death is also greater when a gun is involved , especially in cases of domestic violence . Of course , the choice of weapon could be simply a product of the perpetrator 's focus on killing ; perhaps it 's this intensity , not the choice of weapon , that is really to blame . In light of Wednesday 's knife attacks in Pennsylvania , Zimring 's findings are particularly telling . To separate the effects of the weapon vs. the intent of the perpetrator , he looked at fatality rates for different calibers of guns . If perpetrator intent were really to blame , zeroing in on attacks with guns would `` control '' for that effect . Zimring found , as the instrumentality perspective would predict , that people shot by larger caliber guns -- which carry bigger and more destructive bullets -- were more likely to die than those shot by smaller caliber guns . Adding more evidence to the case that the weapon matters , Zimring and Gordon Hawkins later demonstrated that overall crime rates are n't that much higher in American cities than in comparable cities in other developed countries . We just have higher rates of homicide , and that is because our criminals are more likely to be armed with guns and thus their attacks are more likely to end in the victim 's death . The most important and interesting implication of the instrumentality effect is that if public policy could reduce gun use in crime , the murder rate would go down -- even if the overall crime rate did not . As it turns out , about half of American states have enacted policies that add prison time to felons who use a gun when committing their crimes . These so-called sentencing enhancements , most of which were adopted in the 1970s and 1980s , were intended to reduce the use of guns in violent acts . Scholarly evaluations based on data , not anecdotes , offer some evidence that these policy innovations have been effective . This week 's tragedy ca n't help but invoke memories of the mass shooting at Sandy Hook Elementary School some 16 months ago . The difference today is that , because the Murrysville , Pennsylvania , perpetrator chose to use knives , victims ' families can look forward to a future with their loved ones -- instead of planning their funerals .
Authors : Injury , not death , came from school stabbings in Pennsylvania . They say research has long found for purposes of life and death , the weapon matters . Writers : The knife is a blunt object that can inflict serious injury to someone's body or cause permanent damage if used improperly ( like by an untrained person ). If public policy led to reduction in gun use in crimes , murder rate would go down , they say .