Because negative PMI values tend to be unreliable, we use positive PMI (PPMI), which involves replacing all negative PMI values with zeros (Niwa and Nitta, 1994).
we obtain stance dimensions by applying singular value decomposition to the matrix constructed as follows: Truncated singular value decomposition performs the approximate factorization X ≈ U ΣV , where each row of the matrix U is a k-dimensional description of each stance marker, and each row of V is a k-dimensional description of each subreddit.