The article discusses the uncertainties in determining the efficiency of the automatic regulator rods when measuring the void reactivity coefficient by the method of perturbation of feed water flow at the RBMK-1000 reactor. The existing methods for assessing the effectiveness of a rod are considered. Based on the study of the results of processing measurements of the void reactivity coefficient at the Kursk, Smolensk and Leningrad nuclear power plants, a conclusion is made about the preferred option for processing the experience of weighing the rods of an automatic regulator. The estimation of the instrumental error of measuring reactivity at the power energy level is given.