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Select the method that provides 'the most accurate es!imate of the skew coefficient, Compare the MSE from the isoline map to the MSE for the prediction equati9n. The smaller MSE should then be compared to the variauce of the data. If the MSE is significantly smaller than the variance, the method with the smaller MSE should be used and that MSE used in equation 22 to predict the weighted skew coefficient, for MSEG . If the smaller MSE is not significantly smaller than the variance, neither the isoline map nor the prediction equation provides a more accurate estimate of the skew cgefficient than does the mean value. The mean skew coefficient should be used as it provides the most acpurate estimate and the variance should be used in equation 22 for MSEG . In the absence of detailed studies, the generalized sj(ew coefficient (G) can be read from Plate I , found in the fiyleafpocket of Bulletin 17B. This map of generalized skew coefficients was developed when this bulletin was first introduced and has not been change~. The procedures used to develop the statistical analysis for the individual stations do not conform in all a~pects to the procedures recommended in the , current guide. However, Plate 1 is still considered an alte11fative for use with the guide for those who prefer not to develop their own generalized skew procedures. The accuracy of a regional generalized skew relatiopship is generally not comparable to Plate 1 accuracy. While the average accuracy of Plate 1 is given, the accuracy of subregions within the United States are not given. A comparison should only be made between relationships that cover approximately the same geographical area. Computation of Weighted Skew Coefficient The station and generalized skew coefficient can be: combined to form a better estimate of the skew coefficient for a given watershed. Under the assumption t~at the generalized skew coefficient is unbiased and independent of the station skew coefficient, the MSE :of the weighted estimate is minimized by weighting the station and generalized skew coefficient in inverse proportion to their individual MSE's. This concept is expressed in the following equation adopted from Tasker (1978), which should be used in computing a weighted skew coefficient: MSEG(G) + MSEG (G) Gw = ' MSEG + MSEG (18) where Gw = weighted skew coefficient, G = station skew coefficient, G = generalized skew coefficient, MSEG = mean-square error of generalized skew co~fficient, and MSEG = mean-square error of station skew coeffici~nt. Equation 18 can be used to estimate a weighted sk'1\v coefficient regardless of the source of , generalized skew coefficient, provided the MSE of the generalized skew coefficient can be estimated. When generalized skew coefficients are read from Plate 1, the value of MSEG = 0.302 should be used in equation 18. The MSE of the station skew for log-Pearson Type III random variables can be obtained from the results PEAKFQ 11 DRAFT" 1/30/98