DOUBLE SAMPLING WITH IMPORTANCE SAMPLING TO ELIMINATE BIAS IN TREE VOLUME ESTIMATION OF THE CENTROID METHOD

The bias associated with the centroid method for estimating tree volum e was removed by double sampling using the centroid method in the firs t stage of estimation and importance sampling in the second. The bias and efficiency of various sampling methods were investigated in a numb er of different simulation studies. Three data sets, two of ponderosa pine and the other of mixed species, were used to evaluate the double sampling estimator. We assumed that for small area estimates, such as timber sales, every tree in the population could be visited in the fir st stage and an estimate of bole volume generated using the centroid m ethod. Both simple random and 3P sampling were studied as methods for selecting the second-stage sample trees. For the estimation of volume over a large area, we chose point-3P sampling for applying the double sampling estimator. The bias associated with using the centroid method alone was reduced in every case. No consistent trends for the three d ata sets were found when efficiency was measured in terms of the simul ation variance. The double sampling estimator was the most efficient i n some cases and least efficient in others. (C) 1998 Elsevier Science B.V.