Abstract
Forest inventories are usually compiled without taking into account the existing correlations between sampling units, which is debatable particularly where the calculations involve environmental variables. When the potential correlations between sampling units are overlooked, the accuracy of such inventories becomes distorted in terms of the confidence interval range for the variable of interest, which is volume in cubic meters. The magnitude and form of such distortion will vary according to the correlation intensity between sampling units. This study aimed to present an analysis of the addition of the correlation coefficient to the calculation of the variance of the mean in a systematic sampling procedure of a native forest population or area, as well as its impact on the accuracy of the resulting estimates, with the assumption of independence between sampling units and with the addition of a correlation between sampling units as suggested by Cochran. Results revealed that, where the correlation coefficient was added to the variance of the mean formula, it increased inventory accuracy by about 14.3%, leading to the conclusion that such an effect will occur in any forest inventory being compiled for any forest population or area of interest.
Highlights
Preparing a forest management plan with its relevant procedures is only possible by knowing or by estimating the parameters of the forest population in question.A forest inventory can be defined as an activity that seeks to obtain quantitative and qualitative information on existing forest resources in a preestablished area, a forest inventory consists in partially measuring a population, that is, measuring sampling units or plots to subsequently generate estimates for the total area (LEITE; ANDRADE, 2002)
The estimator proposed by Cochran (1977) can help capture better such relationships by adding the correlation coefficient to the estimator of the variance of the mean
This study aimed to test the effectiveness of the correlation coefficient in improving accuracy of a forest inventory using the systematic sampling procedure, and to compare performances of the traditionally used variance of the mean estimator and the estimator proposed by Cochran (1977)
Summary
A forest inventory can be defined as an activity that seeks to obtain quantitative and qualitative information on existing forest resources in a preestablished area (population), a forest inventory consists in partially measuring a population, that is, measuring sampling units or plots to subsequently generate estimates for the total area (LEITE; ANDRADE, 2002). It estimates the biophysical characteristics of a given forest from direct measurement of individual trees in sampling plots that are representative of the tree population constituting such forest (RODRIGUEZ et al, 2010). It is extremely critical to obtain this necessary information at the lowest possible cost and with the highest possible accuracy (SOARES et al, 2004)
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