Simple random sampling of individual items in the absence of a sampling frame that lists the individuals
BackgroundA 'sampling frame' identifies the sampling units in a population and their locations. It may consist of a listing of sampling units, or it may be based on a map of the population area within which sampling units can be observed. For inventory of large forests or other populations, it is common for no list of individual plants to exist, but it is common to have available a map of the area. When such a map is the only available sampling frame, methods are well established for drawing a simple random sample of fixed area plots. Less well known methods are available if the sample is to consist of individual population members rather than groups of them in plots. Through simulation studies, the efficacies of two methods devised by Dr. K. Iles are considered for drawing a simple random sample of individuals given a map of the population area.FindingsIt is shown that simple random samples of individuals can be drawn satisfactorily using such a map. Further, the estimates obtained from the population mean of individuals, and its precision, are the same as those obtained when a sampling frame consisting of a list of individuals is available. Estimates of the population total can be obtained also, but their precision will be lower than those obtained when a list is available.ConclusionsThe absence of a list of individuals in a population does not preclude simple random sampling of individuals as long as a map of the population area is available. However, a preliminary survey of the population must then be made before sampling starts, and it may be necessary to visit many more sampling units to obtain the required sample than is the case when a list is available. The more complex the spatial arrangement of individuals within the population, the greater will be the number of sampling units that must be visited.
New Zealand Journal of Forestry Science