Stratified Sampling
Stratified sampling involves dividing the site into sections that are more homogenous than the entire area. Boundaries of the sections should be based on factors that are readily identified and mapped, such as different vegetation types, soil types, topography, range sites, range condition classes or utilization levels.
Stratification of the area makes sampling more efficient, because fewer samples are required for a precise estimate of the sample mean and sample variance of a uniform area. Data from each section can be analyzed and interpreted separately, or can be combined to describe the entire management unit. This sampling scheme also overcomes the problem of poor distribution of sample units associated with random sampling.
In stratified sampling, sample size is usually determined for the entire site, and then sample units are divided among the stratified sections. The number of sample units may be allocated on the basis of the area of each section ('proportional allocation'), or by considering variability within each section so that the attribute is estimated with the same precision for all strata ('optimum allocation').
References and Further Reading
Cook, C.W., and J. Stubbendieck. (eds). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp 232-241.