USDA Forest Service Resource Information Group


PlotGEMPlotGEM
Your personal assistant for exploration of sampling alternativesThe question "How much is enough?" is everpresent 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 webbased software application named PlotGEM (Plot Graphics of Estimation of the Mean) to help address sampling issues was developed. PlotGEM can be used to answer many sampling questions, such as:
PlotGEM 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). PlotGEM creates graphs so that the change in estimation accuracy, as sample sizevchanges, can be visualized. PlotGEM 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. PlotGEM 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:
Staff ContactPatrice Janiga, PMP 

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Last modified: Sunday, 24Mar2013 09:49:45 CDT