The experience of manned space missions points to the fact that cosmonaut-operator plays an important role in providing the efficiency and reliability of the crew-spacecraft system. However, the spaceflight factors can have a negative effect on working capacity of crewmembers and give rise to their operational errors. The paper deals with the methodical approach to analysis of the interrelationship of errors and psychophysiological state of cosmonauts as well as the ways of its realization. Described are the system of indices used for expert assessment of the psychophysiological state, peculiarities of work-rest schedule, and intra- and intergroup interaction. Potentialities of this approach are evidenced by results of the analysis of data from one of the MIR missions.