A detailed study using both analytical and numerical calculations of direct and heterodyne differential absorption lidar (DIAL) techniques is conducted to complement previous studies. The DIAL measurement errors depend on key experimental parameters, some of which can be adjusted to minimize the statistical error. Accordingly, the pertinent criteria on optical thickness, the number of photons emitted at the on and off wavelengths, are discussed to reduce the relative error on the total column content or range-resolved measurements that rely on either hard target or atmospheric backscatter returns. In direct detection, the optimal optical thickness decreases from 1.3 to 0.8 when the background increases while the on-line-to-off-line optimal energy ratio decreases from 3.6 to 2.7. In heterodyne detection, the minimum error is obtained for an optical thickness of 1.2 and an energy ratio of 4.3.