I'm trying to calculate the beta of a stock, should I use bias=True or False for covariance? In numpy, bias=True means ddof=0 which means divided by N, when it's the default false it means ddof=1 which means divided by N-1, I've tested both on a bunch of stocks, it matches some of them from yahoo finance, like half and half for each bias True vs False. So I'm not sure which formula I should use.