A sample is that portion of the population which is actually measured. Sampling is a more practical way to obtain inventory or monitoring information because not all members of the population need to be measured. It permits data to be obtained at less effort and expense, and may enhance its integrity by reducing mistakes associated with the tedium of prolonged repetitive measurements.
For example, it would be an unreasonable undertaking to measure every tree constituting the population to obtain information for an inventory on the volume of commercially valuable timber in a National Forest! Instead, data on this attribute may be collected using several smaller areas (perhaps 5 or 10 ha) as samples, and then extrapolated to represent the entire commercial timber resource for the National Forest. Likewise, a Bureau of Land Management field officer is not likely to want to cut and weigh all forage plants within a particular pasture to determine the biomass of forage so that the stocking rate for the next 12 months can be estimated. Instead, they are likely to weigh the forage cut from a sample of smaller quadrats (perhaps each only 1m2), and use this information to make some useful conclusions about forage biomass over the entire pasture.
The key assumption of the sampling process is that sample members make up a representative selection of the entire population. That is, we assume that the information on a particular attribute which we obtained from the sample reliably reflects that attribute in the population. Safeguards incorporated into the sampling process to support this assumption are discussed under Accuracy and Bias and Precision and Error.
References and Further Reading
Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Son, New York, NY. pp 10-11.
Cook, C.W., and J. Stubbendieck. (eds.) 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp 215-219.