with iv the causal effect is estimated by measuring how the outcome varies with the portion of the total variation in the treatment explained by variation in the instrumental variable for example if pas are more likely to be assigned where endemic mammals are present but the presence of endemic mammals only affects deforestation rates through its effect on the likelihood of a parcel s protection then the presence of endemic mammals can be used as an instrument to identify a causal effect of pas on deforestation miteva et al see amin et al for an application in practice it is often hard to find instruments that are both strong related to the intervention and valid unrelated to the outcome