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I <br />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 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 Pb and in the detection <br />well Pd 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 backgrotmd monitoring well, and D and na are the number of detects and the total <br />number of samples in the detection monitoring well. <br />Se = {[(B + D)/(n, + nd)1*[1-( B + D)/(nb + n,)]*[1/ nb + 1/ n,, <br />The test statistic is then calculated as follows: <br />Z= (Pb - P") / S, <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:Interim Final <br />Guidance. PB89-151047. <br />'r... <br />RDDIAPPD4Q97.DOC <br />