USDA Forest Service Resource Information Group
Your personal assistant for exploration of sampling alternatives
The question "How much is enough?" is ever-present when trying to determine how many plots need to be collected to answer managerial questions. In a continuing effort to provide technical assistance to Forest Service personnel designing inventory and monitoring projects or attempting to analyze existing data sets, a web-based software application named Plot-GEM (Plot Graphics of Estimation of the Mean) to help address sampling issues was developed. Plot-GEM can be used to answer many sampling questions, such as:
Plot-GEM can help answer these questions by providing visual displays (graphs) based on calculation of the percent error and thevconfidence interval for the estimate of the means of forest attributes. These calculations can be performed over a range of samplevsizes (also referred to as sampling intensities). Plot-GEM creates graphs so that the change in estimation accuracy, as sample sizevchanges, can be visualized. Plot-GEM also enables the user to explore their options and alternative accuracy objectives byvproviding graphs that depict the confidence intervals associated with different sample sizes. The user can quickly explore "what if" scenarios for different numbers of plot, different suites of variables, and different ways of stratifying their sample.
Plot-GEM uses a bootstrap algorithm to calculate the percent error and confidence interval for the estimates of the means. The bootstrap algorithm is well suited to the plot data situation because it makes no assumptions about the complicated correlation structure that exists among primary sampling units and subplots.
Refer to the following sources for further reading on the advantages of the bootstrap:
Patrice Janiga, PMP
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Last modified: Thursday, 18-Feb-2010 18:11:20 EST