So report some news outlets (e.g., CBS New York), quoting a New York Post story, which likewise makes a similar claim. But the key is in a sentence a bit lower down: “Not all the murder cases have been solved, though, so the number could go higher.”Reading the headline, one is tempted to conclude that 91% (305/334) of murder victims knew their murderer. The reality is that the headline should have been that At Least 46% (152/334) of Victims Knew Their Murderer. That's a lot less dramatic than the original headline.
Could it ever! According to the New York Post story that the CBS story cites, “Police solved 152 homicides in 2013,” out of a total of 334. That means 182 homicides weren’t accounted for, and the total of stranger homicides could thus be anywhere from 29 to 211 (29+182).
I suspect that stranger homicides are more common among the 182 unsolved homicides than they are among the 152 solved ones — in a non-stranger homicide, the killers tend to be easier to identify, precisely because they come from a pool of the victim’s family members, friends, and acquaintances (though note that, as the New York Post mentions, “[a]n acquaintance can include a rival gang member”). The total number of stranger homicides in New York City is thus likely to be a good deal higher to 29, and perhaps closer to 211.
This has greater consequence than simply journalistic sloppiness. If it were true that 91% of murder victims knew their murderer, then it would imply that the city is really safe and you just have to pick your friends carefully (which would be true anyway). 46% of murder victims knowing their murderer reinforces that you have to be vigilant and cannot let your guard down with friends and strangers.
Is this simply an issue of low journalistic standards or is there a reason that the paper and TV want to convey a greater degree of security that actually exists. These days? Who knows.
UPDATE: On reflection, I don't think I elucidated the train of thought enough. The media elected to cast the story in a way that gave the impression that 91% of the time, murder victims know their murderer, whereas the actual numbers tell us that at best all we know is that at least 46% of the time, murder victims know their murderer. We should keep in mind that homicides are a vanishingly small cause of death in the US.
There is probably at least a 70% chance that this misrepresentation is a function of journalist and editorial innumeracy. It happens a lot. But take the alternate hypothesis, that it was deliberately misrepresented. What would explain that? Why would a journalist or editor wish to create the impression that danger comes from one's nearest and dearest rather than from strangers.
Here is my Just So story explanation. If your assailant is most often known to you, then your mitigating strategy is to better select and monitor your friends, family and acquaintances. This creates an illusion of control. In theory, it also creates a powerful community self-policing incentive. If the danger is from people you know, then keep an eye on them, watch out for their well-being. A nice story.
On the other hand, if the danger you face is largely anonymous and random, then you have different mitigating strategies which primarily entail aspects of self-defense such as physical training in a martial art, security systems and guns. Danger from strangers cultivates self-reliance.
Given the well documented political affiliations of journalists, it is easy to see that they are much more likely to wish to push the story that cultivates community rather than guns and self-reliance. So maybe that is the explanation. Sounds too nuanced to me though. I'll go with the 70% probability that it is simply innumeracy.