请问 Baselines PER 描述为什么是 MovieLens- 20M而不是MovieLens- 1M（文章实验是ML-1M上进行的）

PER [39] treats the KG as heterogeneous information networks and extracts meta-path based features to represent
the connectivity between users and items. In this paper,we use manually designed user-item-attribute-item paths
as features, i.e., "user-movie-director-movie", "user-movie-genre-movie", and "user-movie-star-movie" for MovieLens-
20M; "user-book-author-book" and "user-book-genre-book"for Book-Crossing; "user-musician-genre-musician", "user-
musician-country-musician", and "user-musician-age-musician"(age is discretized) for Last.FM. Note that PER cannot be ap-
plied to news recommendation because it’s hard to pre-define meta-paths for entities in news