The theory of Random walkhas been applied to the O-C diagrams of variable stars, and a method has been outlined for the determination of the standard deviation, sigma, of random period-fluctuations, which is free from uncertainties in the expectation value of the period. It is shown, that sigma is an important parameter of variable stars. It seems to be correlated with the rapidity of evolution: the period of a repidly evolving star becomes strongly fluctuating
