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APPENDIX 0 SECTION 1 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 conditioms,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 Pb and in the detection <br /> well P,are calculated.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 /> Se= j[(B+D)/(n,,+nd)]*fl-(B+ D)/(n,+n,)I*fl/ni,+1/n, <br /> The test statistic is then calculated as follows: <br /> z= (Pb—PJ/ Se <br /> The absolute value of Z is compared with 1.96,the 97.5'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:lnterim Final <br /> Guidance. PB89-151047. <br /> RDD(APPD4097= 3 <br />