On The Distribution of Statistics Suitable for Evaluating Rainfall Stimulation Experiments

Two randomization tests of the null hypothesis in cloud seeding experiments are compared–the Wilcoxon-Mann-Whitney test and a test based on an average ratio of seeded to non-seeded amounts of precipitation. Data from the Israeli experiment suggest that the latter test is relatively more sensitive to apparent effects of seeding. The significance level of this test may be estimated by Monte Carlo methods or approximated by using the asymptotic Normal distribution of the average ratio. Sampling trials show that this approximation is adequate only when the experiment is of several years' duration.