An evaluation of sample adequacy for point analysis of ground cover

Abstract

A sample adequacy equation is recommended to calculate the number of point frame samples required for measuring cover on reclaimed land and to assure sample size is adequate to meet Federal rules and regulations. When applied to field data, mining companies and regulatory agencies often find the sample number requirements estimated by the equation are so large that the equation is impractical to use. This equation was studied by sampling with 20, 40, 60, and 100 sets of 10-point frames on 12 X 67-m areas of grazed and ungrazed mixed prairie at Mandan, N.D., and by examining cover data collected with 60 sets of frames from 1.86-ha pastures on reclaimed mined land and native range near Center, N.D. Increasing the number of frames used did not produce more consistent mean values nor did variance decrease. Both total ground cover and bare soil ground cover measure the amount of cover protecting the soil from soil loss; yet, the formula estimated that 1 frame was required to measure total cover whereas 10,086 frames were required to measure bare soil within 10% of the mean with 90% statistical confidence. Histograms suggest cover components that comprise a small percentage of the total ground area are distributed in a Poisson rather than normal fashion; therefore, the equation does not provide a good guide for determining how many samples are required. Another equation, appropriate for binomial and Poisson variates is suggested as a solution to the problem.