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APPENDIX B STATISTICAL METHODS This appendix describes the data analysis methods used to calculate the concentration limits for the monitoring parameters and COCs for LF-1 and LF-2. Tolerance Interval Method The purpose of a tolerance interval approach is to define the concentration limits from background data, within which a large proportion of the monitoring observations should fall with a high probability. The proportion of the population included is referred to as the coverage. The probability with which the tolerance interval includes the proportion of the population is referred to as the tolerance coefficient. Sanitas is a software program that is used to calculate tolerance limits for the Foothill Landfill. Sanitas is a statistical software program. It is specifically designed to evaluate water quality monitoring data for landfills. Sanitas performs all pre- and post-analysis tests required so that the data do not violate size and distribution assumptions of the relevant statistical analysis. Consistent with the USEPA and state recommendations, Sanitas uses a 95 percent coverage and 95 percent tolerance coefficient. The upper 95 percent tolerance limit will contain at least 95 percent of the distribution of observations from background well data. The tolerance interval method is described in the following documents: • Introduction to Statistical Quality Control, D.C. Montgomery. John Wiley Publishing,New York. 1985. • Statistical Analysis of Groundwater Monitoring Data at RCRA Facilities, Interim Final Guidance Document, USEPA. EPA/530-SW-89-026. April 1989. • Statistical Analysis of Groundwater Monitoring Data at RCRA Facilities, Addendum to Interim Final Guidance, USEPA. EPA/530-R-93-003. July 1992. Parametric Tolerance Limit. When conducting the tolerance interval method, Sanitas automatically evaluates the distribution of the data. Within Sanitas, the Coefficient of Variation Test for normality is applied. Sanitas applies the parametric tolerance limit test when the background data set is found to have less than 50 percent nondetects and the background data have a normal or transformed normal distribution. If greater than 15 percent but less than 50 percent of the background data set consist of nondetect values, the mean and standard deviation of the data set are automatically adjusted by Sanitas Foothill WQPS Rev Ldoc B-1