Sample Size

Sample size, or sampling intensity, refers to the number of sample units that will be measured in the inventory or monitoring program.

Sample size is an important consideration during the planning stages of a monitoring program because it is a key way to influence the precision of the collected data. Larger sample sizes improve precision because a greater proportion of the population is being measured. For example, consider a bag of 100 marbles, featuring 8 colors in differing quantities. If we only pull 5 marbles from the bag we don't have a very good idea of colors or quantities, because we haven't even had the opportunity to sample each color (ie., 8 colors cannot be fully represented by 5 marbles). If we pull 40 marbles from the bag we get a better idea of colors and quantities, and our confidence in the representativeness of our data will continue to increase as more marbles from the population in the bag are studied.

Larger sample sizes are required if the population has a large variance or if a species of interest occurs in low abundance. In both cases, a greater proportion of the population must be sampled to obtain an accurate and precise measure of the vegetation attribute.

There are two possible approaches to determine an adequate sample size when designing an inventory or monitoring program.

Sample size is a critical decision in designing an inventory or monitoring program because it is an important way to influence the precision of the sample. Special consideration regarding sample size depends on the vegetation attribute being sampled, and the sampling method selected to collect the data.

References and Further Reading

(Note: pdf files require Adobe Acrobat (free) to view)

Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Son, New York, NY. pp 65-71.

Larson, L.L., and P.A. Larson. 1987. Use of microsite sampling to reduce inventory sample size. Journal of Range Management 40:378-379. (pdf)

Pechanec, J.F, and G. Stewart. 1941. Sagebrush-grass range sampling studies: variability of native vegetation and sampling error. Journal of American Society of Agronomy 33:1057-1071.