In this way, the sample selection, the population is divided into subtypes by layer (Stratum) either without repeating unit is the individual units in a population in which one of the leading landscape landscape layer only. By trying to provide the same chart layer contains units that have similar characteristics as much as possible, and there is a difference between most of the chart floor. When the chart floor to floor sub floor L & nbsp & nbsp & nbsp each floor has a population of N1, N2, ..., and then select a unit of NL, for example, or representative from each layer with tree size n1, n2, ..., nL respectively independently. Therefore, this sample selection method. Need to know the characteristics of the various units in the population so that they can continue to share a chart and set the sample size in each floor chart are 3 characteristics:
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