Sample Design

The inspection plot sample that is used to determine the level of pay must be an unbiased sample that represents the job or pay item as a whole. Plots should cover at least 1 percent of the planting area. When the results are disputed, a 2 percent sample may be needed. Use a fixed-area plot arranged on a systematic grid from a random center point. If inspection plots are installed before the entire unit has been planted, do not make the location of the next plot obvious to the planters. Measuring distance by pacing is usually adequate for locating plots except on steep slopes or in heavy slash; often a tape is useful for measuring distances in these situations. Be sure to adjust the pacing distance based on the slope.

If an inspector follows behind planters and puts in a plot occasionally, the distribution of plots can be biased, with some portions of the unit being oversampled, and other portions are being undersampled. This method may be used to monitor specific planters. It is not suitable for determining overall planting quality because different planters may plant with different levels of quality.

The plot size influences the quality as calculated by the formula. The plot size should be selected to include at least four to six trees on a fully stocked plot. Small plots favor a more even distribution of seedlings. Larger plots allow for more spacing variability, which may be desirable on certain sites. If spacing is important, it may be necessary to specify the plot size in the contract. The number of trees desired per plot to meet the prescribed tree density must be provided as well. Generally, the maximum number of trees on any one plot must not be more than 10 to 15 percent higher than the desired number of trees. Any trees above the maximum number are considered wasted. The contractor may be charged for the wasted trees.