Sampling a Population with Subgroups
When the population we are interested in has subgroups that we are also interested in separately, it is often useful to select a separate sample of elements from each of the subgroups. For such purposes each subgroup is treated as if it were a completely separate population. A sample of whatever size is needed is selected from each of these separate populations, and the values of interest are estimated separately for each population. Suppose that we have reliable information on the locations of all sites in a region. No one has attempted to discover the sizes of these sites, however. We could select a sample of the known sites and go make systematic surface collections in an effort to determine how large they are. These determinations could then form the basis for estimating the mean site size for the region. If, in addition, the region could be divided into three different environmental settings (remnant levees, river bottoms, and slopes) we might be interested in estimating the mean site area for each of the settings.