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• • <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 /> Var(rj) ( 1 )( nw t n - (2) <br /> where: <br /> E w <br /> (3) <br /> w= <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 \ IVar(r)) (4) <br /> where z = the value of the z- statistic (1.645) corresponding to a 90 percent (90 %) <br /> confidence level. <br /> Franchise Agreement for Collection Services with Recology San Mateo County Attachment E -1 <br /> Redwood City Page 8 of 14 <br />