Identification of local causal effects with missing outcome values and an instrument for nonresponse

Even in randomized experiments the identification of causal effects is often threatened by the presence of missing outcome values, with missingness possibly being nonignorable. We provide sufficient conditions under which the availability of a binary instrument for nonresponse allows us to nonparametrically point identify average causal effects in some latent subgroups of units, named Principal Strata, defined by their nonresponse behavior in all possible combinations of treatment and instrument. Examples are provided as possible scenarios where our assumptions may be plausible.