Results of the International Collaborative Study (ICS) show an unexpectedly large scatter of individual dates as well as systematic biases. Very high values of linear correlation coefficients are observed for all results of Stage 2 and for benzene samples of Stage 1. The authors observed moderate correlations for carbonate samples and the lowest for natural samples of wood and peat of Stage 3. The correlation is practicaly negligible among results obtained in different stages. The probable reasons for such effects are seen in medium-term changes in the calibration of the counting systems.