A STATISTICAL METHOD FOR ESTIMATING THE NUMBER OF RATS IN NEW YORK CITY. It is often said that there is one rat in New York City for every human, which would give an estimate of about 8 million rats in the city. This article on the NPR website by Krishnadev Calamur reports on the efforts of a statistician, Jonathan Auerbach, to formulate a more accurate estimate of the number of rats in the city.

Auerbach used the records of the City Health Department of rat sightings by location as reported by phone on 311 during a calendar year. From these data, he estimated the total number of rat-inhabited lots in the City. He then took the average number of rats per rat inhabited lot, as estimated by “pest management professionals who set traps on rat-inhabited lots”. He multiplied his estimate of the number of rat inhabited lots by the estimate of the average number of rats per lot.

Auerbach’s estimate was 2 million rather than 8 million. There seem to be differing opinions, but I think that the 8 million number is close enough to 2 million that the one rat per person claim should not be called an urban legend.