The importance of the Jeffreys Errors Law to the analysis of differences O – C is justified. Based on the law, it is shown that an experiment may be considered as performed correctly only when the differences O – C follow the t-distribution with the number of degrees of freedom v between 5 and 9 or Pearson’s VII type distribution with the index of degree m between 3 and 5 in stable metrological situation. When metrological situation is unstable, the left limit can shift no less than to 3 for v and no less than to 2 for m.