There is then little difficulty in taking a simple random sample. 207) as long as plot-based sampling is to be used. However, if a map of the forest area exists, it can be used as an area sampling frame (Gregoire and Valentine 2008, p. The items making up such populations could be as varied as households within a city, trees in a forest, pebbles on a beach, objects in a photograph, cells on a microscope slide or any other situation where a large number of items appear on a surface or within a volume.įorest (or other natural resource) populations may cover very large areas, too large for a list of individual trees or other plants within it to be compiled. In many areas of human endeavour, it is common to have to deal with large populations for which no list sampling frame has been compiled. If an area sampling frame only is available, the number of sampling units within the population is unknown and it is impossible to know where to start or finish selection of those that are to be included in a sample. It may be a ‘list sampling frame’ whereby a list of each and every sampling unit has been compiled, or it may be an ‘area sampling frame’ that consists only of a map of the area containing the sampling units.
8), before a sample can be drawn from a population, it is necessary to have available a ‘sampling frame’, that is, a mechanism that identifies and locates the sampling units within the population. This work is concerned with difficulties that may be encountered in large populations, such as occur over large forest areas, when it is desired to take a simple random sample consisting of individual items, such as individual trees, from the population.Īs discussed by Gregoire and Valentine ( 2008, p.
Perhaps the most basic method of sampling is ‘simple random sampling’, where each and every member of a population has the same chance of being included in the sample and where all possible samples of a given size have the same chance of selection. 1993 Cochran 1999 Gregoire and Valentine 2008). The more complex the spatial arrangement of individuals within the population, the greater will be the number of sampling units that must be visited.ĭepending on the nature of a population and the information desired through sampling from it, there are many ways in which the sample may be drawn these are discussed in texts on sampling techniques (e.g. 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 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. Estimates of the population total can be obtained also, but their precision will be lower than those obtained when a list is available. 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. It is shown that simple random samples of individuals can be drawn satisfactorily using such a map. Iles are considered for drawing a simple random sample of individuals given a map of the population area. Through simulation studies, the efficacies of two methods devised by Dr. Less well-known methods are available if the sample is to consist of individual population members rather than groups of them in plots. When such a map is the only available sampling frame, methods are well established for drawing a simple random sample of fixed area plots. 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. 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. A ‘sampling frame’ identifies the sampling units in a population and their locations.
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