A frequent critique of the city’s release of value-added ratings for thousands of teachers last week has been that the city has never rated other workers in similar ways.

On Tuesday, Mayor Bloomberg explained the discrepancy, according to Capital New York. In short, Bloomberg said, teachers are not widgets, but other city workers are:

This is not like police and fire. You think about it. Police and fire, we assign a cop or a firefighter to a station, to a post, to a firehouse, to a piece of equipment. And all of the firefighers and all of the cops are changed. Not only are they interchangeable, we deliberately move them around, because that helps their careers and they learn more things and they’re better able to perform their jobs.

Education is different, Bloomberg added. His comments channeled the 2009 “Widget Effect” report by The New Teacher Project, which became fuel for reformers to push tougher teacher evaluations.

“The Widget Effect describes the tendency of school districts to assume classroom effectiveness is the same from teacher to teacher,” the study’s executive summary says. “This decades-old fallacy fosters an environment in which teachers cease to be understood as individual professionals, but rather as interchangeable parts.”