Before revision:  However, it is challenging to train an automatic method for predicting  ATR rehabilitation outcome from treatment data, due to a massive amount of missing data in the data recorded from ATR patients, as well as complex nonlinear relations between measurements and outcomes.
After revision:  However, it is challenging to train an automatic method for predicting ATR rehabilitation outcome from treatment data, due to a massive amount of missing entries in the data recorded from ATR patients, as well as complex nonlinear relations between measurements and outcomes.