Natural resource inventory is an expensive and time-consuming activity and full enumerations, where every tree in the population under investigation is counted and measured, are only done in exceptional cases. A discipline of science evolved around the methods to extract a representative subpopulation for measurement, as a cost effective replacement for full inventories. In essence a sub-sample of trees is selected from the total population and all of the parameters described by the inventory procedure are measured. The mean of the parameters measured for the sub-population is then accepted, with an error margin, as the mean for the population. A discussion on the selection of representative samples follows.