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Peabody Sage Creek Mine April 2012 <br />SL -1 Phase 11 Bond Release Request <br />would not be as affected. Complicating this is the need to continually adjust samples for <br />allowable cover and recalculating for sample adequacy while still sampling in the field. The <br />sagebrush reference area, without the structural characteristics of the mountain brush, does <br />not have this issue and first hit herbaceous cover sample adequacy with reasonable sample <br />sizes is attainable. <br />To address this, an alternative approach was presented to CDRMS in March 2012 by ESCO <br />Associates on behalf of PSCM to establish the first hit herbaceous cover standard for the <br />PSCM mountain brush reference area. Random sampling of the Mountain Brush Reference <br />Area using the prescribed minimum number of samples (15) establishes a reasonable estimate <br />of the variance in the sample universe. For 2010, the estimate of the mean for first -hit <br />herbaceous cover was 12.80 and the standard deviation was 5.10 after fifteen samples had <br />been taken. With these values sample adequacy (sample size sufficient to detect a 10% <br />reduction in the mean with 90% confidence) was not achieved for the Mountain Brush <br />Reference Area (minimum sample size was 29 with a sample size of 15). Were sample <br />adequacy to have been achieved at this point, we would know that the true mean lies <br />somewhere between 11.52 (mean minus 10 %) and 14.12 (mean plus 10 %). Despite lacking <br />sample size sufficient for the upper and lower tails of the 90% confidence range to lie within <br />this ( +1- 10 %) range, we do nonetheless have 90% confidence that the true mean for this <br />population lies between 10.48 and 15.12 percent cover. Figure 9, in Attachment E — <br />Revegetation Supporting Documentation depicts this information. In the development of the <br />cover performance standard here, we use 15.1 (the highest possible value with 90% <br />confidence) as the value for the Mountain Brush Reference Area to be taken forward into the <br />weighting formula prescribed in the permit. For greater detail on the basis and the <br />supporting calculations, please refer to the discussion in the revegetation success evaluation <br />report by ESCO Associates included in Attachment E — Revegetation Supporting <br />Documentation. Particular attention should be made to the text of the report beginning on <br />page 13, Figure 9 and the presentation to CDRMS in Attachment H to this document. <br />Calculation of 90 percent of the cover performance standard appropriate for 2010 sampling is <br />demonstrated below and is based on the following information: <br />9 <br />