Readers can refer to two different variants of Pearson Correlation Coefficient here:
- Correlation Coefficient: Simple Definition, Formula, Easy Steps:
http://www.statisticshowto.com/how-to-compute-pearsons-correlation-coefficients/
- A new user similarity model to improve the accuracy of collaborative filtering:
http://www.sciencedirect.com/science/article/pii/S0950705113003560