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<br />7 <br /> <br />THE BASE METHOD <br /> <br />The Pearson Type III method, as originally presented by H. A. Foster <br />(Ref. 1) in 1924, is described in Bulletin 13 (Ref. 2) in Foster's own <br />words. As used by Foster, the method required the use of the natural <br />data in computations of the mean, standard deviation, and skew co- <br />efficient of the distribution. The current practice, and the recommenda- <br />tion of the Committee, is first to transform the natural data to their <br />logarithms and then to compute the statistical parameters. Because of <br />this transformation the method is now called the log-Pearson Type III <br />method. <br /> <br />Outline of the Method <br /> <br />The following syn1bols are used in the outline submitted by the Work <br />Group on Flood Flow Frequency Analysis, which based its work on the <br />presentation in Bulletin 13: <br /> <br />y = <br />X = <br />N = <br />M = <br />x = <br />S = <br />g = <br />K = <br /> <br />arithmetic magnitude of an annual flood event <br />logarithmic magnitude of Y <br />number of events in the record being used <br />mean of the X's <br />X-M <br />standard deviation of the X's <br />skew coefficient <br />Pearson Type III coordinates expressed in number <br />of standard deviations from the mean for various <br />recurrence intervals or percent chance <br />Q = computed flood flow for a selected recurrence <br />interval or percent chance <br /> <br />The events considered here are flood flows in the annual series. <br />(Definitions of hydrological and statistical terms used here are found <br />in the Glossary of Bulletin 13). In the work, the physical units used <br />for Y (such as cis) are also those for Q. <br />