Hello everyone,   I need to calculate stock beta using the formula: beta= covariance (Rm,Ri) / variance (Rm). I need to calculate beta using 24-60 observations. So, I need to create a new column that has the betas for each date-company calculated depending on the last 24-60 observations (no problem if it started from the current observation). I can create lags but I want more practical solution please giving that my data have several companies. here is how my data looks:   date                 company       Ri      Rm Jan-2001              A                      3%       3.5% Feb-2001              A                      3.2%     3.9% Mar-2001              A                      2.7%     3.1% . . . Jan-2001              B                      3.9%       3.5% Feb-2001              B                      4.1%     3.9% Mar-2001              B                      3.3%     3.1% . .   your help is appreciated. thank you in advance  ... View more