The prediction of rain attenuation statistics is a question of meteorology and probability. Therefore, the possible accuracy of these predictions depends on the knowledge of meteorological mechanisms and data as well as on the statistical uncertainty due to the stochastic nature of meteorological events. Attenuations exceeded for less than 0.1 % of the time within the worst-month are caused by meteorological phenomena which are not well known. Experimental and theoretical investigations show that the inaccuracy is of the same order as the unavoidable statistical uncertainty.