Are there really as many rats in New York as people? Answering that hoary urban legend might not win anybody a Nobel Prize of Statistics, but it's a fascinating topic for a city where rats cause an estimated quarter of all cable-line cuts and unsolved fires.
Jonathan Auerbach is a Ph.D. candidate at Columbia University and the most recent brain to crunch the numbers on New York's rat horde. Past estimates have ranged from an extreme infestation of 28 million whiskered pests to 8.4 million, or the city's current population. That last estimate reflects the popular "one rat per person" ideology, which is shaky at best, according to Snopes:
The "one rat per person" claim stems from a study of rats conducted in England by W.R. Boelter and published in 1909 under the title The Rat Problem. Boelter surveyed the English countryside (but not villages, towns, or cities) and came up with an educated guess, estimating that England had one rat per acre of cultivated land. Since England had 40 million acres of cultivated land at the time, Boelter pegged the country's rat population at 40 million. And since England also had a human population of 40 million at the time, there was some basis for claiming that the country was host to one rat per person.
Auerbach's research drives another nail into this myth's coffin. He estimated the ratty multitide using a thoroughly modern method—delving into open data, in this case the Department of Health's trove of 311 calls for rodent sightings. Auerbach counted the distribution of rat alerts among New York's roughly842,000 property lots, and then ran a statistical analysis to extrapolate population numbers. His conclusion: The rat threat has been greatly exaggerated, as there are probably only about 2 million whiskered critters in the city.
And even that number could be inflated, he claims in the journal Significance:
Indeed, our lot comparison method likely overestimates the population of rats in NYC. This is because we treat every lot with at least one rat sighting as evidence that a full colony of rats inhabited the lot for the duration of the study period. However, it is possible that the territory of one rat colony encompasses several lots. Since we do not know the extent to which a rat colony has established itself on each reported lot, we proceed as though each reported lot is associated with a full colony of rats
So there it is: maybe only a couple million rats. That's no doubt a comforting supposition for folks with musophobia, maybe not so much for somebody who just had one pop up in a toilet or gnaw a sleeping child's face.