Selected Sampling
Selected sampling involves active selection of members of the population that are considered to be most representative of the objectives outlined in the inventory or monitoring program. For example, key areas are often selected for closer attention in utilization studies, because it is not possible to monitor utilization across the entire site. Likewise, in a riparian monitoring program, transects may be subjectively located at several locations considered typical of the riparian zone.
The potential of selected sites to accurately represent the larger area depends upon personal judgement. With experience and skill, people are often able to successfully select representative sites. However, in other cases, personal bias or unrecognized natural factors affecting the area may confound the accuracy of the data from the selected sites. Therefore, extra care must be taken in interpreting the data, in addition to the initial site selection.
Fewer sites are usually chosen in a selected sampling design, because they are assumed to provide a good representation of the population, and no measure of sample variance is required. As a consequence, the overall sampling effort is greatly reduced, and selected sampling is often adopted in monitoring programs where the primary goal is to investigate change over time for extensive areas. However, because the data provides no measure of variability for the area being sampled, sample precision cannot be estimated and statistical inference analysis cannot be applied to describe spatial patterns fro the general area. However, it is possible to use statistical inference methods to compare values for each site among the various times that the sample units were measured.
References and Further Reading
Greig-Smith, P. 1983. Quantitative plant ecology. Blackwell Scientific Publications, Oxford.3rd ed. p 20.
Kent, M., and P. Choker. 1992. Vegetation description and analysis. Belhaven Press, London. pp 40-44.
Smith, E.L., and G.B. Ruyle. 1991. Considerations when Monitoring Rangeland Vegetation. In: G.B. Ruyle. (ed). Some methods for monitoring rangelands and other natural area vegetation. University of Arizona, College of Agriculture, Extension Report 9043. pp 2-3.