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<br />Population <br /> <br />ReauI'X'enae <br />Interval (Return <br />Period, Exaeed- <br />anae Interval) <br /> <br />Sample <br /> <br />Skew Coefficient <br /> <br />Standard <br />Deviation <br /> <br />... <br /> <br />The entire (usually infinite) number of data from <br />which a sample is taken or collected. The total <br />number of past, present, and future floods at a <br />location on a river is the population of floods for <br />that location even if the floods are not measured or <br />recorded. <br /> <br />The average time interval between actual occur- <br />rences of a hydrological event of a given or greater <br />magnitude. In an annual flood series, the average <br />interval in which a flood of a given size is <br />exceeded as an annual maximum. In a partial <br />duration series, the average interval between floods <br />of a given size, regardless of their relationship to <br />the year or any other period of time. The dis- <br />tinction holds even though for large floods recurrence <br />intervals are nearly the same for both series. <br /> <br />An element, part, or fragment of a "population." <br />Every hydrologic record is a sample of a much <br />longer record. <br /> <br />See "coefficient of skewness." <br /> <br />A measure of the dispersion or precision of a <br />series of statistical values such as precipitation or <br />streamflow. It is the square root of the sum of <br />squares of the deviations from the arithmetic mean <br />divided by the number of values or events in the <br />series. It is now standard practice in statistics <br />to divide by the number of values minus one in order <br />to get an unbiased estimate of the variance from the <br />sample data. <br /> <br />2-4 <br />