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2.5 Sample Adeauacy Determination <br />Sampling within each managerial unit was conducted to a minimum of 20, 30, or 45 samples per <br />unit. From these preliminary efforts, sample means and standard deviations for total non-overlapping <br />vegetation ground cover, production, and woody plant density were calculated. For non-monitoring <br />applications, the typical procedure is that sampling must continue until an adequate sample, nmin, has <br />been collected in accordance with the Cochran formula (below) for determining sample adequacy, <br />whereby the population is estimated to within 10% of the true mean (N) with 90% confidence. <br />When the inequality (nmin <_. n) is true, sampling is deemed adequate; and nmin is determined as <br />follows: <br />nmin = (t252) / (0.1 X )2 <br />where: n = the number of actual samples collected (initial size = 20, 30, or 45) <br />t = the value from the one-tailed t distribution for 90% confidence with n-1 <br />degrees of freedom; <br />s 2 = the variance of the estimate as calculated from the initial samples; <br />x = the mean of the estimate as calculated from the initial samples. <br />If the initial samples do not provide a suitable estimate of the mean (i.e., the inequality is false), <br />additional samples would be collected until the inequality (nn,in 5 n) becomes true. <br />CEDAR CREEK ASSOCIATES, Inc. Page 8 Colowyo Coal Company <br />2008 Phase III Bond Release Evaluation