“…Data-driven approaches, which are based on statistical learning methods, provide useful and practical tools for model reduction. The past decades witness revolutionary developments of data-driven strategies, ranging from parametric models (see, e.g., [ 8 , 9 , 10 , 11 , 12 , 13 , 14 ] and the references therein) to nonparametric and machine learning methods (see, e.g., [ 15 , 16 , 17 , 18 ]). These developments demand a systematic understanding of model reduction from the perspectives of dynamical systems (see, e.g., [ 7 , 19 , 20 ]), numerical approximation [ 21 , 22 ], and statistical learning [ 17 , 23 ].…”