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e Appendix 9 Section Description t tiI Analyses ,.w RDDIAPPM97.Doc The statistical analyses used to determine if a discharge from the landfill is impacting groundwater in the detection monitoring wells were selected in accordance with EPA guidance (EPA, 1989) and State of California Regulations (23 CCR 2550). The procedure is outlined in Figure D-1 and each step is discussed below. Identification of Outliers The test for outliers is to determine if there is statistical evidence that an observation appears extreme and does not fit the distribution of the remainder of the data. Any observations identified as outliers were assigned a value of "O" in the Lab Flag field in the database and were not used for statistical analysis. To identify outliers, the statistic T. was calculated as shown below, where Xn is the largest observation, X is the mean of the observations, and S is the standard deviation. Tn ; (Xn - XVS The statistic T„ is compared with a critical value for the sample size (EPA, 1989). The sample was identified as an outlier if it met three conditions: (1) the statistic exceeds the critical value, (2) the observation is at least four times greater than the mean, and (3) the observation was at least four times greater than the next highest values. Test for Normality The statistical procedures recommended in the EPA guidance and California regulations are robust with respect to departures from many of the normal distribution assumptions (EPA, 1989). Extensive study of the distribution is not considered necessary unless a nonparametric method of analysis.is selected. The coefficient of variation was used to determine if the assumption of a normal distribution was appropriate. The coefficient of variation is the standard deviation, S, divided by the mean of the observations, X. If the coefficient of variation exceeds one, there is evidence that the data are not normal and the assumption of a normal distribution is not - appropriate for the data set. be distributed, the residuals for the data set will be If the data set appears to not normally checked for normality. If these are normal, the distribution of the data set will be assumed normal and the parameteric test will be selected. If the residuals are not normally distributed, the data set will be transformed and checked for normality. If the transformed data set is normal, the parametric procedure will be selected. If the transformed data set is M not normal, a nonparametric procedure will be used for the data set. ,.w RDDIAPPM97.Doc