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<br />Major problems in flood frequency analysis at gaged locations are <br />encountered when making flood estimates for probabilities more rare than <br />defined by the available record. For these situations the guide describes <br />the information to incorporate in the analysis but allows considerable <br />latitude in analysis. <br /> <br />B. Plotting Position <br />Calculations specified in this guide do not require designation of <br />a plotting position. Section V.C.l., describing treatment of historic <br />data, states that the results of the analysis should be shown graphically <br />to permit an evaluation of the effect on the analysis of including <br />historic data. The merits of alternative plotting position formulae <br />were not studied and no recommendation is made. <br />A general formula for computing plotting positions (23) is <br />P = (m-a) <br />(N-a-b+ 1) (6) <br /> <br />where <br /> <br />m = the ordered sequence of flood values with <br />the largest equal to 1 <br />N = Number of items in data set and a and b depend <br />upon the distribution. For symmetrical <br />distribution a=b and the formula reduces to <br />P = (m-a) <br />(N-2a+l) (7) <br />The Weibull plotting position in which a in equation 7 equals 0 was <br />used to illustrate use of the historic adjustment on Figure 6-3 and has <br />been incorporated in the computer program, Appendix 13, to facilitate <br />data and analysis comparisons by the program user. This plotting position <br />was used because it is analytically simple, and intuitively easily <br />understood (18, 24). <br /> <br />Weibull Plotting Position formula: <br /> <br />p = <br /> <br />m <br />N + 1 <br /> <br />(8) <br /> <br />24 <br />