http://content.time.com/time/photogallery/0,29307,1968182_2081048,00.html
An unsettling urban legend claims that there are as many rats as people in New York City. But that's simply not true, according to a statistician who found that a generous estimate for the rat population would actually be 2 million — far fewer than New York's 8 million humans.
To estimate animal populations, ecologists often use a "capture–recapture" method. First, researchers capture and mark a random sample of a species, and then later, they round up a second random sample of the animals. Based on the percentage of marked animals recaptured in the second batch, ecologists can estimate the species' total population in a given area. But Jonathan Auerbach, a doctoral student in the statistics department at Columbia University, said that the city's Department of Health and Mental Hygiene was not keen on having a large-scale rodent-releasing experiment in New York.
Instead, Auerbach turned to a wealth of data on rat sightings collected by the 311 information and service hotline, where New Yorkers can file complaints about noise or rodents, get updates on parking regulations, or even request a tree to be planted on their block.