The process of developing a standard to be used for subjective estimation of activity has been studied. Subjects were asked to go on assessing the size of an experimental object until they were completely sure of the correctness of assessment. It has been found out that in the process subjects first develop an orienting, standard, than a preliminary standard, and in the long run a final standard with which every single measurement is compared. During performance of the task the standard can be modified following the dynamics of the subjective idea of the "real" size. The data obtained makes it possible to conclude that consecutive results of an assessment series are standard-dependent and do not merely represent random oscillation round a certain mean value, which must not be ignored in professional estimation-practice.