More advanced methods, such as GloVe, set non-uniform weights for observed entries to reflect their confidence.
the time complexity of their algorithm is proportional to number of nonzero weights (|(i, j) | C ij = 0|), thus they have to set zero weights for all the unobserved entries (C ij = 0 for Ω − ), or try to incorporate a small set of unobserved entries by negative sampling.