APE assumes the availability of source texts (S ip ), corresponding MT output (T mt ) and the human post-edited (T pe ) version of T mt , and APE systems can be modelled as an MT system between S ip T mt (i.e., a joint representation of S ip and T mt ) and T pe .
statistical APE (SAPE) systems can also be built without the availability of S ip using only sufficient amounts of parallel "target-side" T mt -T pe text within the statistical MT (SMT) framework.