Based on complaints to the 311 line, you would assume the majority of illegal conversions take place in lower Manhattan. If you plot the complaints on a map, lower Manhattan "blows up like a tomato," Flowers says. But actually, inspectors from the Buildings Department are far more likely to find the illegal conversions in the outer boroughs: Brooklyn, Queens and the Bronx.

"We're sending our resources where the complaints are instead of where the conditions are," Flowers says. "And those conditions are very serious. Why are they serious? In the spring and summer of 2011, we had two buildings go up in flames that had been illegally converted. We had some firemen get seriously injured and we had people die."

Indeed, buildings that are illegal conversions are the scenes of a lot more fire per property, Flowers says. And, more importantly, firemen are much more likely to be injured or killed fighting such fires because exits tend to get blocked.

"My office was tasked with figuring out how to fix that," Flowers says.

Successful Data Project Starts with Talking to People