Eighteen routes across two councils in Northern Ireland were assigned to either receive the door hanger or not, and door hangers were placed on door knobs after rubbish was collected. We measured the total tonnage of landfill rubbish and recyclables per route for about six weeks before the intervention and again for five weeks after the intervention for both the routes that received the door hanger and the routes that didn’t.
Black bin rubbish decreased for both the control routes and the routes with the door hanger. The routes with the intervention did not decrease landfill rubbish statistically more than the control routes. The same pattern was found for brown bin/garden waste and blue bin/recyclables.
The evidence does not suggest that the door hangers were an effective intervention in increasing recycling. Without a comparison group, however, it may have been concluded that the intervention was a success, leading to a waste of time and money for the councils. Through an experiment with a control group, however, we can evaluate many different iterations of a design and find which works best, while discarding ones that do not have the intended effect.