The method of matching gets rid of selection bias due to observables. It is not able to address bias caused by un-oservables. Similarly, in the context of an agri-environment scheme, treated farmers can be compared to similar untreated farmers that have the same observed characteristics (e.g., same education level, age, farm structure and equipment but who differ in treatment status). It is then possible to form the difference in outcomes between the treated farmer and his twin.