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APPENDIX 0 SECTION I DESCRIPTION OF STATISTICAL ANALYSES <br /> The corrected mean and standard deviation are then used to determine the tolerance limit <br /> as described previously. <br /> This test statistic is not useful if the mean is slightly larger than the detection limit or if there <br /> are multiple reported detection limits.When these conditions interfered with the <br /> application of Cohen's method,the nondetect values were replaced with one-half the <br /> detection limit and the tolerance interval was calculated. <br /> Test of Proportions <br /> If less than 50 percent but not fewer than 10 percent of the values are above the detection <br /> limit, a test of proportions was used to compare the background data with the detection <br /> well data.The test determines if the proportion of detects in the detection well differs <br /> significantly from the proportion in the background well..If there is a significant difference, <br /> there is statistically significant evidence of contamination.This test requires at least five <br /> detected values and five nondetects. <br /> To perform the test,the proportion of detects in the background well Pbandin the detection <br /> well Pdarecalculated.The standard error of the difference in proportions is calculated as <br /> shown below,where B and N are the number of detects and the total number of samples in <br /> the background monitoring well,and D and n,are the number of detects and the total <br /> number of samples in the detection monitoring well. <br /> S,.= ([(B+D)/(N+nd)]*[l-(B+D)/(nb+nd)1*11/nb+I/ n, <br /> The test statistic is then calculated as follows- <br /> z=(Pb-PI)/ S, <br /> The absolute value of Z is compared with 1.96,the 97.54,percentile from the standard <br /> normal distribution.If the value exceeds 1.96,there is statistically significant evidence at the <br /> 5 percent significance level that the proportion of detects in the detection monitoring well <br /> exceeds the proportion in the background monitoring well. <br /> References <br /> United States Environmental Protection Agency,Office of Solid Waste.April 1989. <br /> Statistical Analysis of Ground-Water Monitoring Data at RCRA Facilities:Interim Final <br /> Guidance. PB89-151047. <br /> RDD(APPD4497.DOC 3 <br />