Within the last five years, my former student (now colleague) Elias Bareinboim and I have succeeded in giving a complete criterion for deciding when results are transportable and when they are not. As usual, the proviso for using this criterion is that you represent the salient features of the data-generating process with a causal diagram, marked with locations of potential disparities. “Transporting” a result does not necessarily mean taking it at face value and applying it to the new environment. The researcher may have to recalibrate it to allow for disparities between the two environments.
