Specific Guidelines on Sample Size for Frequency
Sample size is a critical decision in determining frequency, because it is a key way to influence the precision of the collected data. Larger sample sizes are required if the collected data has a large sample variance or if the key species occurs in low abundance. In both cases, a greater proportion of the population must be sampled to obtain an accurate and precise measure of frequency.
From a practical sense, remember that frequency is determined as a percentage. Therefore, if 50 quadrats are recorded, frequency increases in 2% intervals for each quadrat the species is found, so that the outcome can be no more precise than ±1%. For this reason, 100 quadrats are often accepted as the minimum sample size for frequency data, and 200 quadrats is recommended as a sampling intensity that provides a reasonable compromise between statistical and logistical considerations. Many more samples (e.g., 500-1000 quadrats) would be needed if the sampling program aims to detect small changes in frequency.
Also refer to Sample Size in the Sampling Concepts section.
Sample Size for Frequency Sampling
- 100 quadrats is the minimum sample size.
- frequency data from randomly located quadrats usually follows a binomial distribution, so that the conventional statistical technique cannot be used to estimate sample size.
- a sample size of 100 or 200 quadrats allows the statistical analysis of frequency data based on readily available Binomial Statistics Tables.
- much larger sample sizes are required to confidently detect small changes between sampling times or sites.
References and Further Reading
(Note: pdf files require Adobe Acrobat (free) to view)
Despain, D.W., Ogden, P.R., and E.L. Smith. 1991. Plant frequency sampling for monitoring rangelands. In: G.B. Ruyle. (ed). Some methods for monitoring rangelands and other natural area vegetation. University of Arizona, College of Agriculture, Extension Report 9043. p. 13.
Hironaka, M. 1985. Frequency approaches to monitor rangeland vegetation. In: W.C. Krueger. (chairman). Proceeding 38th Annual Meeting, Society for Range Management, Salt Lake City, UT, February 1985. p. 85.
Mueller-Dombois, D., and H. Ellenburg. 1974. Aims and methods of vegetation ecology. John Wiley Sons, New York, NY. pp. 74-76.
Whysong, G.L., and W.W. Brady. 1987. Frequency sampling and type II errors. Journal of Range Management 40:472-474. (pdf)