Daily operating data from South Lake Tahoe covering six years of continuous operation together with shorter records from other plants have been analyzed. The data are skewed but can be normalized by conversion to logs, thus permitting estimates of long-term reliability based on normal statistics. A spread factor, S, which is the antilog of the standard deviation is shown to be a useful parameter for estimating future performance. Spread factors lie between 1.5 and 2.0 for all the parameters studied except MPN. The Ely, Minnesota, plant on Shagawa Lake produces an effluent that has approximately one tenth the concentration of phosphates obtained by other AWT plants, but the spread factor is about the same for all plants.