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Hoshin V. Gupta; Harald Kling; Koray K. Yilmaz; Guillermo F. Martinez. (2009). Decomposition of the mean squared error and NSE performance criteria: Implications for improving hydrological modelling. , 377(1-2), 80–91. doi:10.1016/j.jhydrol.2009.08.003 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research