The urban school was located in a part of the city in which levels of sulfur dioxide in the air were usually far lower than is typical for urban areas.

At both schools, the plants in the greenhouses grew much more quickly than did plants planted outdoors in plots near the greenhouses.

The urban class conducting the experiment was larger than the rural class conducting the experiment.

Heavy vehicular traffic such as is found in cities constantly deposits grime on greenhouse windows, reducing the amount of light that reaches the plants inside.

Because of the higher levels of sulfur dioxide in the air at the urban school, the air filters for the urban school's greenhouse were changed more frequently than were those at the rural school.