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Attachment E-1 <br />Contamination Measurement Methodology: Single Loads <br />The confidence interval for this estimate will be derived in two (2) steps. First, the <br />variance around the estimate will be calculated, accounting for the fact that the ratio <br />included two (2) random variables (the component and total sample weights). The <br />variance of the ratio estimator equation follows: <br />where: <br />Var(rj)-(��t11 <br />I <br />� Cy -Y W, <br />n-1 <br />(2) <br />(3) <br />(For more information regarding Equation 2, please refer to Sampling Techniques, 3rd <br />Edition by William G. Cochran [John Wiley & Sons, Inc., 1977].) <br />Second, precision levels at the 90 percent (90%) confidence level will be calculated for a <br />component's mean as follows: <br />r ±(z Var(r )) <br />(4) <br />where z = the value of the z -statistic (1.645) corresponding to a 90 percent (90%) <br />confidence level. <br />Collection Services Franchise Agreement <br />with Recology San Mateo County <br />City of Redwood City <br />AT1110AG21EOLW54/Recology of San Mateo <br />Attachment E-1 <br />Contamination Measurement Methodology <br />Page 8 of 14 <br />