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2.3 Sample Adequacy Determination <br />Sampling within each reference area and Phase II Bond Release block area was conducted to a <br />minimum of 20 or 72 samples, respectively. From these preliminary efforts, sample means and standard <br />deviations for total non -overlapping vegetation ground cover were calculated. For non -monitoring <br />applications, the typical procedure is that sampling continues until an adequate sample, nmin, has been <br />collected in accordance with the Cochran formula (below) for determining sample adequacy, whereby the <br />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 = (t 2S 2) / (0.1 X )2 <br />where: n = the number of actual samples collected (initial size = 20 or 72) <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 <br />of the mean (i.e., the inequality is false), additional <br />samples would be collected until the inequality (nmin <_ <br />n) becomes true. <br />r� <br />Table SA - Sample Adequacy Results <br />Reference Areas <br />Phase II Bond <br />Release Block <br />Mountain Sagebrush <br />Shrub <br />Mean = <br />48.83 <br />64.40 <br />51.95 <br />Variance = <br />188.06 <br />134.67 <br />204.58 <br />n = <br />72 <br />20 <br />20 <br />nm�, = <br />13.20 <br />5.72 <br />13.36 <br />