Sample Unit Size

The optimum sample unit size for rangeland sampling depends on the attribute being described, the size of plants present, and the scale of spatial patterns within the vegetation. Sometimes the size selected in a rangeland inventory or monitoring program is determined by convention or past practices. In fact, it is important to continue using sample units of the same size for repeated measurements. Therefore, sample unit size must be carefully considered in the planning stages because of its critical role in determining sample accuracy and sample precision.

Sample unit size influences sample accuracy by controlling the likelihood of boundary decisions. Larger sample units have a lower perimeter:area ratio (Table 1), which reduces possible bias from incorrect boundary decisions.

Table 1. Effect of size on perimeter:area ratios of sample units
Dimensions Perimeter:Area Ratio
1 x 1 4.0
2 x 2 2.0
3 x 3 1.3
4 x 4 1.0
5 x 5 0.8
6 x 6 0.7

Sampling precision is influenced by manipulating sample unit size in a manner that considers vegetation patterns, so that more variability is encompassed within sample units rather than among sample units. Larger sample units reduce sample variance and usually generate data that more closely follow a normal distribution. However, these advantages are offset by several other factors. First, the size of the sample units must more than double to reduce sample size by 50%, causing an increase in the total area sampled. Second, the smaller sample size suggested by a lower sample variance would decrease the precision of the sample, due to its effect on the t-value when calculating confidence intervals. Third, sample units with a large area are difficult to methodically count or estimate, which increases the possibility of bias. Finally, the logistical constraints of time and resources generally permit fewer samples to be taken using large sample units, presenting problems of achieving an adequate dispersal of sample units across the site. In practice, the trade-off between fewer, large sample units and many smaller sample units depends on the time taken to measure attributes at each sample unit and the time needed to travel to and locate the additional sample units.

In summary, sample units should be large enough so that few are completely empty, and so that most include more than one plant. Conversely, inefficient expenditure of time relative to the additional information acquired is apparent in sample units that are larger than necessary. Larger sample units are also needed to accommodate larger life forms, such as bigger shrubs and trees. In contrast, smaller sample units are more convenient in dense vegetation. A system of nested quadrats can overcome problems faced when sampling species of different life form or abundance.

References and Further Reading

Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Son, New York, NY. pp 33-35.

Cook, C.W., and J. Stubbendieck. (eds). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp 220-221.

Daubenmire, R. 1968. Plant communities: A textbook on plant synecology. Harper Row, New York, NY. pp 85-89.