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If we have: User 1, rated product A with 0 star. User 2, rated product A with 0 star.

What is the Pearson's correlation coefficient or Cosine Similarity between them? According to the formula, it should be 0/0. But what is 0/0? It is not a number(NaN). If both rated the same product with the same rating, should the correlation be 1, which is perfect similarity.

Minh Hoang
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  • What do you mean by `Pearson Correlation`? – Hovo May 30 '15 at 14:26
  • Oh, I changed. It should be Pearson's correlation coefficient. – Minh Hoang May 30 '15 at 14:30
  • The pearson correlation coefficient is not a measurement for similarity. It measures the degree of linear dependence between several points. If you look at the formula, you see that you basically normalize the covariance by the product of the standard deviations of x and y - So pearson correlation between two scalars is undefined. If you want to calculate a distance from it, you have to take that into account. – cel May 30 '15 at 15:05

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