The power of FIA Phase 3 Crown-Indicator variables to detect changeThis article is part of a larger document. View the larger document here.
The goal of Phase 3 Detection Monitoring as implemented by the Forest Inventory and Analysis Program is to identify forest ecosystems where conditions might be deteriorating in subtle ways over large areas. At the relatively sparse sampling intensity of the Phase 3 plot network, a rough measure of success for the forest health indicators developed for this purpose is the ability to detect meaningful change when indicator data are pooled across two or three States. The statistical power of a test is the probability of detecting a difference of a certain magnitude when it indeed exists, and is defined as 1 minus the type II error (i.e., 1-beta). This study applies statistical power analysis to Phase 3 Crown-Indicator variables (crown density, foliage transparency, and crown dieback) to determine how many plots are necessary to detect various degrees of change at various levels of statistical power.