While Manhattan's best scientists are still trying to figure out why rats love roaming through our legs and our faces, one rat researcher is just trying to establish how many rats there are playing house in our streets and our baked goods. Statistician Jonathan Auerbach attempted to compile a NYC rat census to estimate how many rats really are here—and he discovered that the urban myth that there is a 1:1 ratio of rats to humans is probably false.
You can read Auerbach's full essay about his methodologies and findings at Significance [PDF], but suffice to say, it took a lot of trial and error to come to his conclusions. Turns out that rats aren't very good about responding to surveys; he turned to a method called capture-recapture estimation, which also proved problematic. But combining that method with 311 rat data, focusing on the locations where rats reside, was his breakthrough.
"The analysis classified rat sightings by city lot, of which there are roughly 842,000 in New York City," it says in a press release. "The researchers estimated 40,500 rat-inhabited lots in the city. By liberally assuming that 40 to 50 rats belong to a typical colony and that one full colony occupies each rat-inhabited lot, the researchers concluded that 2 million would be an extremely generous estimate of the city's rat population."
Here's some more specifics about how he explained it:
Pest management professionals who set traps on rat-inhabited lots can estimate the average number of rats per inhabited lot, and in order to estimate the number of rat-inhabited lots in NYC, we will follow steps similar to capture-recapture. For reference purposes, let us call this adapted procedure 'lot comparison.'
We first observe the number of lots that reported a rat sighting during the first half of 2010. These lots constitute our first sample and are our "marked" lots. Then we observe the number of lots that reported a rat sighting during the first half of 2011. These lots constitute our second sample. Some of them are "marked" in that they were also identified within our first sample. A "marked" lot that appears in the second sample period has been 'recaptured'. If we assume that a recaptured lot is as likely to be reported as any other rat-inhabited lot, the proportion of "recaptured" lots in the second sample period will then provide an estimate of the total number of rat-inhabited lots.
Other outlets have looked at Auerbach's data and breathed a sigh of relief—it's so much less than we feared, they say. But let's not lose the forest for the trees: he still established that there are literally millions of rats in NYC, millions of rats who eat through cars and pastries without prejudice. This is no consolation: this is just two million feral disease-carrying boogeymen who can't be reasoned with. And we know we're going to keep sleeping with that pitchfork under our pillow.