| Abstract: | 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. |
