between_max             Compute the maximum between-group
between_min             Compute the minimum between-group
between_sd              Compute the standard deviation of between-group
within_max              Compute the maximum within-group for a panel
                        data
within_min              Compute the minimum within-group for panel data
within_sd               Compute the standard deviation of within-group
                        for a panel data
xtsum                   Calculate summary statistics for panel data
