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4-92 <br />• 4.3.4 Reference Area Selection <br />Reference areas were established for each vegetation type <br />affected by disturbance. Reference areas were selected <br />following the completion and analysis of cover data for the <br />affected vegetation types. This procedure permitted the <br />integration of cover data into the reference area selection <br />process. Factors considered in the location and selection <br />of representative reference sites included similarity of <br />vegetation and habitat of the reference area compared to the <br />affected area. Important vegetation factors evaluated were <br />structure, composition, species dominance as expressed by <br />cover, production, and density. Prominent abiotic habitat <br />• factors included soil type, topography, slope, and aspect. <br />Following collection and analysis of data the comparability <br />of reference areas and analogous affected areas was evaluated <br />statistically for cover and production with the formula <br />'t L <br />h• nv <br />where xl = mean cover or production for the affected area <br />x2 = mean cover or production for the reference area <br />t = a t-statistic value, which varies depending on <br />desired level of confidence and sampling size <br />si = the variance of cover and production for the <br />affected area <br />s2 = the variance of cover and production for the <br />. reference area <br />nl = sample size in the affected area <br />n2 = sample size in the reference area. <br />