The goal of this study was to identify relationships between sampling effort, using a point frame, and the precision of estimates of vegetative cover in sagebrush steppe. Data for 208 point-frame samples were randomly drawn and 95% confidence intervals were identified for varying sample sizes of individual species and species groups. Confidence interval widths decreased rapidly as sample sizes increased from 1 to 50, and more slowly as sample sizes were increased further. For the more abundant species, sample sizes of fewer than 50 frames were sufficient to reduce the 95% confidence interval to less than one-half of average cover. For species with average cover less than 5%, the sample size required to achieve the same relative confidence was much larger. Analyses were repeated to compare point frames with 36 versus 50 sample points per frame. Using a point frame with 50 sample points decreased the number of frames required to achieve the same level of confidence; however, the reduction in sample size was less than the 28% reduction in sample points per frame. Because of the time required to establish transects and position the point frame, it may still be worth sampling more points per frame.
Inouye, Richard S.
"Sampling effort and vegetative cover estimates in sagebrush steppe,"
Western North American Naturalist: Vol. 62
, Article 12.
Available at: http://scholarsarchive.byu.edu/wnan/vol62/iss3/12