Sources of Sampling Error
Some error may arise from boundary decisions, due to the absolute 'present-or-absent' feature associated with frequency. This is particularly so for species occurring at lower frequencies, where an individual falling near the boundary is probably the only individual representing that species in the quadrat. With more abundant species, other individuals found within the quadrat negate the problem of deciding how to treat the individual at the edge of the quadrat.
The other source of error is associated with identification of the species within the quadrat. Overlooking certain species (particularly if small), or species misidentification, will compromise the accuracy of frequency data.
References and Further Reading:
Cook, C.W., and J. Stubbendieck. (eds). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. p. 63.