By doing this for all treated farmers and taking the average of these differences, the average causal effect of the AES on participating farmers (i.e. the average effect of treatment on the treated - ATT) can be estimated (OECD, 2012). Stated otherwise, matching assumes that similarity in the observed characteristics translates into similarity in unobservable characteristics, correlated with the outcome and the biodiversity policy assignment, or that such unobservables are negligible sources of bias (Miteva, 2012). There may also be unobservables, as mentioned earlier, such as motivation or environmental awareness that may influence e.g., a farmer’s decision to participate in an AES.