Methods and apparatus for including obtaining raw surface data for present values of torque (TTD) generated by a top drive operably coupled with a drillstring, obtaining raw surface data for present values of rotational speed (ω) of the top drive corresponding to the TTD, obtaining inertia (JTD) of the top drive, and estimating torque TST of the drillstring based on the obtained TTD data, the obtained data, and the obtained JTD data. The estimated drillstring torque TST may be utilized to determine a surface torque oscillation performance index (STOPI).