title: How to speed up calculating variance among sparse matrix
body: <p>Thank you for your help. Let me explain my question more clearly.</p>
  
  <p>I have several coo_matrix M with same shape. Indexes of each matrix represent a journal pair. Each element represents the frequency of a journal pair. For example,
  for all matrices, [1,2] gives the frequency of journal pair 1-2; [2,1] gives the frequency of journal pair 2-1.</p>
  
  <p>My goal is to get the variance of frequency for each journal pair.</p>